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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2017	Nov. 01, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	Five9, Inc.
Trading Symbol	FIVN
Entity Central Index Key	0001288847
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		55,650,970

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 63,364	$ 58,122
Accounts receivable, net	17,231	13,881
Prepaid expenses and other current assets	4,809	3,008
Total current assets	85,404	75,011
Property and equipment, net	17,958	14,688
Intangible assets, net	1,190	1,539
Goodwill	11,798	11,798
Other assets	2,365	2,203
Total assets	118,715	105,239
Current liabilities:
Accounts payable	4,787	3,366
Accrued and other current liabilities	11,967	9,604
Accrued federal fees	1,146	2,742
Sales tax liability	1,174	1,347
Notes payable	486	742
Capital leases	6,057	6,230
Deferred revenue	13,699	10,047
Total current liabilities	39,316	34,078
Revolving line of credit	32,594	32,594
Sales tax liability — less current portion	1,207	1,476
Notes payable — less current portion	0	318
Capital leases — less current portion	6,867	5,915
Other long-term liabilities	959	530
Total liabilities	80,943	74,911
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000 shares authorized, no shares issued and outstanding at September 30, 2017 and December 31, 2016	0	0
Common stock, $0.001 par value; 450,000 shares authorized, 55,631 shares and 53,363 shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	56	53
Additional paid-in capital	212,505	196,555
Accumulated deficit	(174,789)	(166,280)
Total stockholders’ equity	37,772	30,328
Total liabilities and stockholders’ equity	$ 118,715	$ 105,239

Condensed Consolidated Balance Sheets - Parenthetical - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred Stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred Stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred Stock, shares issued (in shares)	0	0
Preferred Stock, shares outstanding (in shares)	0	0
Common Stock, par value (in USD per share)	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	450,000,000	450,000,000
Common Stock, shares issued (in shares)	55,631,423	53,363,013
Common Stock, shares outstanding (in shares)	55,631,423	53,363,013

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 50,081	$ 40,982	$ 144,822	$ 117,883
Cost of revenue	20,497	17,790	60,741	51,164
Gross profit	29,584	23,192	84,081	66,719
Operating expenses:
Research and development	6,689	6,041	20,372	17,642
Sales and marketing	16,502	12,925	49,212	38,268
General and administrative	4,679	6,143	20,384	18,561
Total operating expenses	27,870	25,109	89,968	74,471
Income (loss) from operations	1,714	(1,917)	(5,887)	(7,752)
Other income (expense), net:
Extinguishment of debt	0	(1,026)	0	(1,026)
Interest expense	(865)	(961)	(2,635)	(3,357)
Interest income and other	118	12	326	(66)
Total other expense, net	(747)	(1,975)	(2,309)	(4,449)
Income (loss) before income taxes	967	(3,892)	(8,196)	(12,201)
Provision for (benefit from) income taxes	43	(2)	142	68
Net income (loss)	$ 924	$ (3,890)	$ (8,338)	$ (12,269)
Net income (loss) per share:
Basic (in USD per share)	$ 0.02	$ (0.07)	$ (0.15)	$ (0.24)
Diluted (in USD per share)	$ 0.02	$ (0.07)	$ (0.15)	$ (0.24)
Shares used in computing net income (loss) per share:
Basic (in shares)	55,310	52,708	54,579	52,078
Diluted (in shares)	59,441	52,708	54,579	52,078
Comprehensive Income (Loss):
Net income (loss) and comprehensive income (loss)	$ 924	$ (3,890)	$ (8,338)	$ (12,269)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net loss	$ (8,338)	$ (12,269)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization	6,246	6,302
Provision for doubtful accounts	66	58
Stock-based compensation	10,703	6,927
Loss on extinguishment of debt	0	1,026
Reversal of interest and penalties on accrued federal fees	(2,133)	0
Non-cash adjustment on investment	(233)	0
Amortization of debt discount and issuance costs	60	221
Accretion of interest	16	11
Others	(50)	(9)
Changes in operating assets and liabilities:
Accounts receivable	(3,406)	(2,383)
Prepaid expenses and other current assets	(1,861)	(1,927)
Other assets	71	(25)
Accounts payable	1,409	1,039
Accrued and other current liabilities	1,774	2,749
Accrued federal fees and sales tax liability	95	(90)
Deferred revenue	3,676	2,449
Other liabilities	131	(75)
Net cash provided by operating activities	8,226	4,004
Cash flows from investing activities:
Purchases of property and equipment	(1,809)	(973)
Increase in restricted cash	0	(60)
Net cash used in investing activities	(1,809)	(1,033)
Cash flows from financing activities:
Proceeds from exercise of common stock options	3,280	4,050
Proceeds from sale of common stock under ESPP	1,800	792
Proceeds from revolving line of credit	0	32,594
Repayments on revolving line of credit	0	(12,500)
Repayments of notes payable	(547)	(23,866)
Payments of capital leases	(5,708)	(4,618)
Payment of prepayment penalty and related fees	0	(368)
Payments for debt issuance costs	0	(206)
Net cash used in financing activities	(1,175)	(4,122)
Net increase (decrease) in cash and cash equivalents	5,242	(1,151)
Cash and cash equivalents:
Beginning of period	58,122	58,484
End of period	63,364	57,333
Supplemental disclosures of cash flow data:
Cash paid for interest	2,588	3,430
Cash paid for income taxes	113	86
Non-cash investing and financing activities:
Equipment obtained under capital lease	7,482	5,548
Equipment purchased and unpaid at period-end	$ 22	$ 13

Description of Business and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business and Summary of Significant Accounting Policies	Description of Business and Summary of Significant Accounting Policies Five9, Inc. and its wholly-owned subsidiaries (the “Company”) is a provider of cloud software for contact centers. The Company was incorporated in Delaware in 2001 and is headquartered in San Ramon, California. The Company has offices in Europe and Asia, which primarily provide research, development, sales, marketing, and client support services. Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. Therefore, these condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016. In the opinion of management, the condensed consolidated financial statements reflect all adjustments, which are normal and recurring in nature, necessary for fair financial statement presentation. All intercompany transactions and balances have been eliminated in consolidation. Certain prior period amounts included in the condensed consolidated financial statements have been reclassified to conform to the current period presentation. Use of Estimates The preparation of condensed consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. The significant estimates made by management affect revenue, the allowance for doubtful accounts, intangible assets, goodwill, loss contingencies, including the Company’s accrual for federal fees and sales tax liability, accrued liabilities, stock-based compensation, provision for income taxes and uncertain tax positions. Management periodically evaluates such estimates and they are adjusted prospectively based upon such periodic evaluation. Actual results could differ from those estimates. Significant Accounting Policies The Company’s significant accounting policies are disclosed in its Annual Report on Form 10-K for the year ended December 31, 2016. During the nine months ended September 30, 2017, there were no significant changes to the Company's significant accounting policies. Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. This ASU simplifies several aspects of the accounting for share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. The guidance was effective for the Company beginning in the first quarter of 2017. Accordingly, commencing in January 2017, the Company accounted for forfeitures as they occurred, rather than by estimating expected forfeitures. The net effect of this change was recognized as a $0.2 million reduction to accumulated deficit in the condensed consolidated financial statements. Upon adoption of the new standard, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) are recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards are treated as discrete items in the reporting period in which they occur. The Company also recognizes excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. The Company has applied the modified retrospective adoption approach beginning January 1, 2017 and prior periods have not been adjusted. As a result, the Company established a net operating loss deferred tax asset of $5.3 million to account for prior period excess tax benefits through retained earnings, however an offsetting valuation allowance of $5.3 million was also established through retained earnings because it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there is no impact on the Company’s condensed consolidated financial statements. Recent Accounting Pronouncements Not Yet Effective In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires measurement and recognition of expected credit losses for certain types of financial assets held. ASU 2016-13 is effective for the Company in its first quarter of 2020, and earlier adoption is permitted beginning in the first quarter of 2019. The Company is currently evaluating the impact of ASU 2016-13 on its consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). Under the new guidance, a lessee will be required to recognize assets and liabilities for both finance, or capital, and operating leases with lease terms of more than 12 months. The ASU also will require disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. Lessor accounting will remain largely unchanged from current GAAP. In transition, lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach that includes a number of optional practical expedients that entities may elect to apply. This guidance is effective for the Company beginning in the first quarter of 2019. Early adoption is permitted. The Company is currently assessing the effect the guidance will have on its consolidated financial statements. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers: Topic 606 and issued subsequent amendments to the initial guidance in August 2015, March 2016, April 2016 and May 2016 within ASU 2015-04, ASU 2016-08, ASU 2016-10 and ASU 2016-12, respectively (ASU 2014-09, ASU 2015-04, ASU 2016-08, ASU 2016-10 and ASU 2016-12 collectively, Topic 606). Topic 606 requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers and will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. Topic 606 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than are required under existing GAAP. Topic 606 is effective for the Company's annual and interim reporting periods beginning January 1, 2018 using either a retrospective or a cumulative effect transition method. The Company has developed an implementation plan to adopt this new guidance based on the cumulative effect transition method. As part of this plan, the Company is currently assessing the impact of the new guidance on its results of operations. Based on procedures performed to date, the Company does not believe the adoption of Topic 606 will have a material impact on the timing of revenue, but the adoption will have a material impact on commission expense. The Company will continue to evaluate this assessment in 2017. The Company intends to adopt Topic 606 on January 1, 2018.

Fair Value Measurements

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

The Company carries cash equivalents consisting of money market funds at fair value on a recurring basis. Fair value is based on the price that would be received from selling an asset in an orderly transaction between market participants at the measurement date. Fair value is estimated by applying the following hierarchy, which prioritizes the inputs used to measure fair value into three levels and bases the categorization within the hierarchy upon the lowest level of input that is available and significant to the fair value measurement:

Level 1 — Observable inputs which include unadjusted quoted prices in active markets for identical assets.

Level 2 — Observable inputs other than Level 1 inputs, such as quoted prices for similar assets, quoted prices for identical or similar assets in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset.

Level 3 — Unobservable inputs that are supported by little or no market activity and that are based on management’s assumptions, including fair value measurements determined by using pricing models, discounted cash flow methodologies or similar techniques.

The fair value of assets carried at fair value was determined using the following inputs (in thousands):



	September 30, 2017
	Total		Level 1		Level 3
Assets
Cash equivalents
Money market funds	$	20,060	$	20,060	$	—
Other Assets
Embedded conversion option held for investment	$	946	$	—	$	946

	December 31, 2016
	Total		Level 1		Level 3
Assets
Cash equivalents
Money market funds	$	20,069	$	20,069	$	—
Other Assets
Embedded conversion option held for investment	$	873	$	—	$	873

Fair Value Measured and Recorded Using Significant Unobservable Inputs (Level 3) (in thousands):



	September 30, 2017
Beginning balance	$	873
Total gains included in earnings	73
Ending balance	$	946

The valuation of an embedded conversion option held for investment with a convertible note was performed using a Black-Scholes option-pricing model which relies primarily on estimates of expected term, volatility, risk-free rate, and dividends related to our investment in a privately-held company, or the investee. The most significant unobservable inputs used in the determination of estimated fair value of the option are the estimates of share price and volatility, driven by the investee's ability to meet financial targets, and which directly correlates to the fair value recognized in other non-current assets within the condensed consolidated balance sheets.

The fair value of this asset is estimated quarterly by management based on inputs received from the investee's management using the excess earnings method under the income approach. Potential valuation adjustments are made as the progress toward achieving financial targets becomes determinable, with the impact of such adjustments being recorded to 'Interest income and other' in our condensed consolidated statements of operations and comprehensive loss.

During the nine months ended September 30, 2017, there were no transfers in or out of Level 3 from other levels in the fair value hierarchy.

There were no assets or liabilities measured at fair value on a non-recurring basis as of September 30, 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Financial Statement Components

9 Months Ended

Sep. 30, 2017

Financial Statement Components [Abstract]

Financial Statement Components

Cash and cash equivalents consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Cash	$	43,304	$	38,053
Money market funds	20,060		20,069
Cash and cash equivalents	$	63,364	$	58,122

Accounts receivable, net consisted of the following (in thousands):



	September 30, 2017			December 31, 2016
Trade accounts receivable	$	16,096		$	12,640
Unbilled trade accounts receivable, net of advance client deposits	1,152			1,253
Allowance for doubtful accounts	(17		)	(12		)
Accounts receivable, net	$	17,231		$	13,881

Prepaid expenses and other current assets consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Prepaid expenses	$	3,034	$	2,199
Other current assets	1,775		809
Prepaid expenses and other current assets	$	4,809	$	3,008

Property and equipment, net consisted of the following (in thousands):



	September 30, 2017			December 31, 2016
Computer and network equipment	$	44,517		$	37,664
Computer software	6,383			5,133
Internal-use software development costs	500			475
Furniture and fixtures	1,223			1,130
Leasehold improvements	641			624
Property and equipment	53,264			45,026
Accumulated depreciation and amortization	(35,306		)	(30,338		)
Property and equipment, net	$	17,958		$	14,688

In accordance with our property and equipment policy, we review the estimated useful lives of our fixed assets on an ongoing basis. A review of our existing estimates indicated that the actual lives of certain data center assets were longer than previously estimated useful lives used for depreciation purposes in our financial statements. As a result, effective July 1, 2017, we changed the estimated useful lives of certain data center assets to better reflect the estimated periods during which these assets will remain in service. The estimated useful lives of these assets, which we previously depreciated for three years, have now been increased to a range of three to five years. This change in accounting estimate decreased depreciation expense by $0.8 million for the three and nine months ended September 30, 2017, increased both basic and diluted net income per share by $0.02 for the three months ended September 30, 2017, and decreased both basic and diluted net loss per share by $0.02 for the nine months ended September 30, 2017.

Depreciation and amortization expense associated with property and equipment was $1.8 million and $5.9 million for the three and nine months ended September 30, 2017, respectively, and $2.0 million and $5.9 million for the three and nine months ended September 30, 2016, respectively.

Property and equipment capitalized under capital lease obligations consist primarily of computer and network equipment and was as follows (in thousands):



	September 30, 2017			December 31, 2016
Gross	$	44,471		$	35,504
Less: accumulated depreciation and amortization	(29,072		)	(23,128		)
Total	$	15,399		$	12,376

Accrued and other current liabilities consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Accrued compensation and benefits	$	9,669	$	7,456
Accrued expenses	2,298		2,148
Accrued and other current liabilities	$	11,967	$	9,604

Intangible Assets

9 Months Ended

Sep. 30, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Intangible Assets

The components of intangible assets were as follows (in thousands):



	September 30, 2017							December 31, 2016
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Developed technology	$	2,460	$	(1,390	)	$	1,070	$	2,460	$	(1,126	)	$	1,334
Customer relationships	520		(411		)	109		520		(333		)	187
Domain names	50		(39		)	11		50		(32		)	18
Non-compete agreements	140		(140		)	—		140		(140		)	—
Total	$	3,170	$	(1,980	)	$	1,190	$	3,170	$	(1,631	)	$	1,539

Amortization expense related to intangible assets was $0.1 million and $0.3 million for the three and nine months ended September 30, 2017, respectively, and $0.1 million and $0.4 million for the three and nine months ended September 30, 2016, respectively.

As of September 30, 2017, the expected future amortization expense for intangible assets was as follows (in thousands):



Period	Expected Future Amortization Expense
2017	$	117
2018	442
2019	351
2020	280
Total	$	1,190

Debt

9 Months Ended

Sep. 30, 2017

Debt Disclosure [Abstract]

Debt

2016 Loan and Security Agreement

On August 1, 2016, or the Effective Date, the Company entered into a loan and security agreement, or the 2016 Loan and Security Agreement, with the lenders party thereto and City National Bank, as agent for such lenders. The 2016 Loan and Security Agreement provides for a revolving line of credit, or the New Revolving Credit Facility, of up to $50.0 million and matures on August 1, 2019. On the Effective Date, the Company borrowed $32.6 million under the 2016 Loan and Security Agreement. The proceeds were used to extinguish existing indebtedness under all prior Loan and Security Agreements and for working capital and other general corporate purposes.

Loans under the 2016 Loan and Security Agreement bear a variable annual interest rate of the prime rate plus 0.50%, subject to a 0.25% increase if the Company's adjusted EBITDA is negative at the end of any fiscal quarter. The Company has agreed to pay a fee of 0.25% per annum on the unused portion of the New Revolving Credit Facility as well as an anniversary fee of $31,250 on each of the first and second anniversaries of the Effective Date. The Company is accreting the total estimation of unused fees and anniversary fees evenly over the full term of the 2016 Loan and Security Agreement. Under the terms of the 2016 Loan and Security Agreement, the outstanding balance cannot exceed the Company’s trailing four months of MRR (monthly recurring revenue including subscription and usage) multiplied by the average trailing 12 month dollar based retention rate (calculated on the same basis as in the Company’s periodic reports filed with the SEC). As of September 30, 2017, the outstanding principal balance under the 2016 Loan and Security Agreement was $32.6 million, which is included in 'Revolving line of credit' in the condensed consolidated balance sheets. As of September 30, 2017, the amount available for additional borrowings was $17.4 million.

The Company incurred approximately $0.2 million in fees that were directly attributable to the issuance of this credit facility. These costs are deferred and included within 'Prepaid expenses and other current assets' and 'Other assets' in the Company's condensed consolidated balance sheets and being amortized to interest expense on a straight-line basis over three years starting from the Effective Date of the New Revolving Credit Facility.

The obligations of the Company under the 2016 Loan and Security Agreement are guaranteed by the Company’s subsidiary, Five9 Acquisition. The Company’s obligations under the 2016 Loan and Security Agreement and Five9 Acquisition’s obligations under its guaranty are secured by a first priority perfected security interest in and lien on substantially all of the Company’s and Five9 Acquisition's assets. The 2016 Loan and Security Agreement contains certain customary covenants, including the requirement that the Company maintain $25.0 million of unrestricted cash deposited with the lenders for the term of the agreement, a minimum liquidity ratio of unrestricted cash and accounts receivable to the outstanding amounts under the 2016 Loan and Security Agreement, as well as customary events of default. Under the 2016 Loan and Security Agreement, the Company is also prohibited from declaring dividends or making other distributions on capital stock. The Company was in compliance with these covenants as of September 30, 2017.

The Company recorded a $1.0 million loss on extinguishment of debt in the third quarter of 2016 under the 2013 Loan and Security Agreement and the 2014 Loan and Security Agreement (each as described below). The loss was comprised of $0.4 million in prepayment penalties, a $0.4 million write-off of unamortized debt discounts, and a $0.2 million write-off of unamortized debt issuance costs.

2013 Loan and Security Agreement

Prior to entering into the 2016 Loan and Security Agreement on August 1, 2016, the Company had a revolving line of credit of up to $20.0 million, or the Prior Revolving Credit Facility, under a loan and security agreement with a lender, which was entered into in March 2013 and last amended in December 2014, or the 2013 Loan and Security Agreement. The Prior Revolving Credit Facility carried a variable annual interest rate of the prime rate plus 0.50% and would have matured on December 1, 2016.

The 2013 Loan and Security Agreement was collateralized by substantially all the assets of the Company. The balance outstanding could not exceed the lesser of (i) $20.0 million or (ii) an amount equal to the Company’s monthly recurring revenue for the three months prior multiplied by the average Dollar-Based Retention Rate over the prior twelve months, less the amount accrued for the Company’s Universal Service Fund (“USF”) obligation (accrued federal fees). As of August 1, 2016, the Company had canceled and paid back all amounts due under the Prior Revolving Credit Facility.

In connection with its acquisition of SoCoCare in October 2013, the Company also borrowed $5.0 million under a term loan, or the Term Loan, under the 2013 Loan and Security Agreement in October 2013. Monthly interest-only payments were due on the advance at the prime rate plus 1.50% through September 2014. Principal and interest payments were due in equal monthly installments from October 2014 through the maturity of the Term Loan in March 2017. As of August 1, 2016, the Company had canceled and paid back all amounts due under the Term Loan.

The 2013 Loan and Security Agreement contained certain covenants, including the requirement that the Company maintain $7.5 million of cash deposited with the lender for the term of the 2013 Loan and Security Agreement. The Company was in compliance with these covenants through the cancellation date of August 1, 2016. The 2013 Loan and Security Agreement remained senior to other debt, including the debt issued under the 2014 Loan and Security Agreement discussed below.

2014 Loan and Security Agreement

Prior to entering into the 2016 Loan and Security Agreement on August 1, 2016, the Company had a term loan facility of $30.0 million with a syndicate of two lenders, or Lenders, which was entered into in February 2014 and amended in December 2014 and February 2015, or the 2014 Loan and Security Agreement. The term loan facility was available to the Company in tranches. The first tranche for $20.0 million was advanced upon entering into the agreement. The remaining $10.0 million was available for drawdown by the Company until February 20, 2016 in $1.0 million increments, which expired on February 20, 2016. The Company incurred $0.4 million in debt costs in connection with borrowing the first tranche in February 2014. The term loan bore interest at a variable per annum rate equal to the greater of 10% or LIBOR plus 9%. Interest was due and payable on the last business day of each month during the term of the loan commencing in February 2014. Monthly principal payments were due beginning in February 2016 based on 1/60th of the outstanding balance at that time and would continue until all remaining principal outstanding under the term loan became due and payable in February 2019. As of August 1, 2016, the Company had canceled and paid back all borrowings under the 2014 Loan and Security Agreement.

The term loan was secured by substantially all the assets of the Company and was subordinate to the 2013 Loan and Security Agreement. The 2014 Loan and Security Agreement contained certain covenants and included the occurrence of a material adverse event, as defined in the agreement and determined by the Lenders, as an event of default. The Company was in compliance with these covenants through the effective cancellation date of August 1, 2016.

In connection with entering into the 2014 Loan and Security Agreement, the Company issued to the Lenders warrants to purchase 177,865 shares of common stock at $10.12 per share, which vest and become exercisable over a ten year term from the date of issuance, based on amounts drawn under the $30.0 million term loan facility. Based on the drawdown of $20.0 million in February 2014, 118,577 shares of common stock issuable under the warrants vested and are exercisable by the Lenders. The fair value of these vested warrants of $1.0 million was recorded as a discount against the debt proceeds and was being recognized as additional interest expense over the term of the loan. The remaining 59,288 shares of common stock issuable under the warrants pertaining to the undrawn $10.0 million did not vest and were no longer exercisable on February 20, 2016 when the $10.0 million was no longer available for borrowing.

Promissory Note

In July 2013, the Company issued a promissory note to the Universal Service Administrative Company, or USAC, for $4.1 million in principal amount as a financing arrangement for that amount of accrued federal fees. The promissory note carried a fixed annual interest rate of 12.75% and was repayable in 42 equal monthly installments of principal and interest beginning in August 2013. As of December 31, 2016, approximately $0.1 million of this promissory note was outstanding and is included as notes payable in the accompanying condensed consolidated balance sheets. This promissory note was fully paid as of January 2017.

FCC Civil Penalty

In June 2015, the Company entered into a consent decree with the Federal Communications Commission, or FCC, Enforcement Bureau, in which the Company agreed to pay a civil penalty of $2.0 million to the U.S. Treasury in twelve equal quarterly installments starting in July 2015 without interest. As a result, the Company discounted the $2.0 million liability, which was accrued in the third quarter of 2014 for the then tentative civil penalty, to its present value of $1.7 million at an annual interest rate of 12.75% to reflect the imputed interest and reclassified this discounted liability from 'Accrued federal fees' to 'Notes payable.' The $0.3 million discount was recorded as a reduction to general and administrative expense in the three months ended June 30, 2015 and is being recognized as interest expense over the payment term of the civil penalty. As of September 30, 2017 and December 31, 2016, the outstanding civil penalty payable was $0.5 million and $1.0 million, respectively, of which the net carrying value was $0.5 million and $0.9 million, respectively, and is included as 'Notes payable' in the accompanying condensed consolidated balance sheets.

As of September 30, 2017 and December 31, 2016, the Company’s outstanding debt is summarized as follows (in thousands):



	September 30, 2017			December 31, 2016
Promissory note to USAC	$	—		$	120
FCC civil penalty	500			1,000
Total notes payable, gross	500			1,120
Less: discount	(18		)	(79		)
Total notes payable, net carrying value	482			1,041
Revolving line of credit	32,594			32,594
Interest accretion under 2016 line of credit	4			19
Total debt, net carrying value	$	33,080		$	33,654
Less: current portion of debt *	$	(486	)	$	(742	)
Total debt, less current portion **	$	32,594		$	32,912


* Included in ‘Notes payable’ in the condensed consolidated balance sheets.
** Included in ‘Notes payable - less current portion’ and 'Revolving line of credit — less current portion' in the condensed consolidated balance sheets.

Maturities of the Company’s outstanding debt as of September 30, 2017 are as follows (in thousands):



Period	Amount to Mature
2017	$	167
2018	333
2019	32,594
Total	$	33,094

Stockholders' Equity

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Stockholders' Equity

Stockholders’ Equity

Capital Structure

The Company is authorized to issue 450,000,000 shares of common stock with a par value of $0.001 per share. As of September 30, 2017 and December 31, 2016, the Company had 55,631,423 and 53,363,013 shares of common stock issued and outstanding, respectively.

The Company is also authorized to designate and issue up to 5,000,000 shares of preferred stock with a par value of $0.001 per share in one or more series without stockholder approval and to fix the rights, preferences, privileges and restrictions thereof. As of September 30, 2017 and December 31, 2016, the Company had no shares of preferred stock issued and outstanding.

Warrants

As of September 30, 2017 and December 31, 2016, the Company had outstanding warrants to purchase 13,013 and 131,597 shares of common stock, respectively, with a weighted-average exercise price of $5.76 per share and $9.80 per share, respectively. The warrants outstanding as of September 30, 2017 will expire on October 18, 2023.

Common Stock Reserved for Future Issuance

Shares of common stock reserved for future issuance related to outstanding equity awards, common stock warrants, and employee equity incentive plans were as follows (in thousands):



		September 30, 2017
Stock options outstanding		4,597
Restricted stock units outstanding		2,133
Shares available for future grant under 2014 Plan		7,526
Shares available for future issuance under ESPP		1,495
Common stock warrants outstanding		13
Total shares of common stock reserved		15,764

Equity Incentive Plans

Prior to its initial public offering (“IPO”), the Company granted stock options under its Amended and Restated 2004 Equity Incentive Plan, as amended (the “2004 Plan”).

In March 2014, the Company’s board of directors and stockholders approved the 2014 Equity Incentive Plan (“2014 Plan”) and 5,300,000 shares of common stock were reserved for issuance under the 2014 Plan. In addition, on the first day of each year beginning in 2015 and ending in 2024, the 2014 Plan provides for an annual automatic increase to the shares reserved for issuance in an amount equal to 5% of the total number of shares outstanding on December 31st of the preceding calendar year or a lesser number as determined by the Company’s board of directors. Pursuant to the automatic annual increase, 2,668,150 and 2,558,231 additional shares were reserved under the 2014 Plan on January 1, 2017 and 2016, respectively.

No further grants were made under the 2004 Plan once the 2014 Plan became effective on April 3, 2014. Upon the effectiveness of the 2014 Plan, all shares reserved for future issuance under the 2004 Plan became available for issuance under the 2014 Plan. After the IPO, any forfeited or expired shares that would have otherwise returned to the 2004 Plan instead return to the 2014 Plan.

The 2004 Plan and the 2014 Plan are described in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016.

Stock Options

A summary of the Company’s stock option activity during the nine months ended September 30, 2017 is as follows (in thousands, except years and per share data):



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2016	5,556		$	5.23
Options granted (weighted average grant date fair value of $8.20 per share)	511		16.83
Options exercised	(1,336	)	2.46
Options forfeited or expired	(134	)	12.64
Outstanding as of September 30, 2017	4,597		$	7.10	6.3	$	77,235

The Company has computed the aggregate intrinsic value amounts disclosed in the above table based on the difference between the exercise price of the options and the closing market price of the Company’s common stock of $23.90 per share as of September 30, 2017 for all in-the-money options outstanding.

Restricted Stock Units

A summary of the Company's restricted stock unit ("RSU") activity during the nine months ended September 30, 2017 is as follows (in thousands, except per share data):



	Number of Shares		Weighted Average Grant Date Fair Value Per Share
Outstanding as of December 31, 2016	2,019		$	7.65
RSUs granted	1,022		17.31
RSUs vested and released	(745	)	8.68
RSUs forfeited	(163	)	10.92
Outstanding as of September 30, 2017	2,133		$	11.62

Employee Stock Purchase Plan

The Company's 2014 Employee Stock Purchase Plan ("ESPP") became effective on April 3, 2014 and is described in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016. The number of shares of common stock originally reserved for issuance under the ESPP was 880,000 shares, which will increase automatically each year, beginning on January 1, 2015 and continuing through January 1, 2024, by the lesser of (i) 1% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year; (ii) 1,000,000 shares of common stock (subject to adjustment to reflect any split or combination of the Company’s common stock); or (iii) such lesser number as determined by the Company’s board of directors. Pursuant to the automatic annual increase, 533,630 and 511,646 additional shares were reserved under the ESPP on January 1, 2017 and 2016, respectively.

One purchase event occurred during the nine months ended September 30, 2017 as follows: 141,222 shares were purchased on May 15, 2017 at a price of $12.75 per share under the ESPP.

Stock-Based Compensation

Stock-based compensation expenses for the three and nine months ended September 30, 2017 and 2016 were as follows (in thousands):



	Three Months Ended				Nine Months Ended
	September 30, 2017		September 30, 2016		September 30, 2017		September 30, 2016
Cost of revenue	$	599	$	357	$	1,608	$	951
Research and development	797		547		2,235		1,510
Sales and marketing	1,084		626		3,236		1,604
General and administrative	1,240		989		3,624		2,862
Total stock-based compensation	$	3,720	$	2,519	$	10,703	$	6,927

As of September 30, 2017, unrecognized stock-based compensation expenses by award type and their expected weighted-average recognition periods are summarized in the following table (in thousands, except years).



	Stock Option		RSU		ESPP
Unrecognized stock-based compensation expense	$	6,461	$	23,056	$	181
Weighted-average amortization period	2.8 years		2.9 years		0.1 years

The Company recognizes stock-based compensation expense that is calculated based upon awards that have vested, reduced for actual forfeitures. All stock-based compensation for equity awards granted to employees and non-employee directors is measured based on the grant date fair value of the award.

The Company values RSUs at the closing market price of its common stock on the date of grant. The Company estimates the fair value of each stock option and purchase right under the ESPP granted to employees on the date of grant using the Black-Scholes option-pricing model and using the assumptions noted in the below table. The weighted-average assumptions used to value stock options granted during the three and nine months ended September 30, 2017 and 2016 were as follows:



Stock Options	Three Months Ended		Nine Months Ended
	September 30, 2017	September 30, 2016	September 30, 2017	September 30, 2016
Expected term (years)	6.1	6.1	6.2	6.0
Volatility	53%	48%	49%	48%
Risk-free interest rate	2.0%	1.2%	2.0%	1.5%
Dividend yield	—	—	—	—

Net Income (Loss) Per Share

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Net Income (Loss) Per Share

Basic net income (loss) per share is calculated by dividing net income (loss) by the weighted average number of common shares outstanding during the period, and excludes any dilutive effects of employee stock-based awards and warrants. Diluted net income (loss) per share is computed giving effect to all potentially dilutive common shares, including common stock issuable upon exercise of stock options and warrants and vesting of restricted stock units.

The following table presents the calculation of basic and diluted net income (loss) per share (in thousands, except per share data).



	Three Months Ended					Nine Months Ended
	September 30, 2017		September 30, 2016			September 30, 2017			September 30, 2016
Net income (loss)	$	924	$	(3,890	)	$	(8,338	)	$	(12,269	)
Weighted-average shares of common stock outstanding	55,310		52,708			54,579			52,078
Basic net income (loss) per share	$	0.02	$	(0.07	)	$	(0.15	)	$	(0.24	)

Net income (loss)	$	924	$	(3,890	)	$	(8,338	)	$	(12,269	)
Weighted-average shares of common stock outstanding	55,310		52,708			54,579			52,078
Effect of dilutive shares	4,131		—			—			—
Weighted average shares of common stock outstanding	59,441		52,708			54,579			52,078
Diluted net income (loss) per share	$	0.02	$	(0.07	)	$	(0.15	)	$	(0.24	)

The following securities were excluded from the calculation of diluted net income (loss) per share because their effect would have been anti-dilutive (in thousands).



	Three Months Ended		Nine Months Ended
	September 30, 2017	September 30, 2016	September 30, 2017	September 30, 2016
Stock options	471	5,626	4,597	5,626
Restricted stock units	29	2,096	2,133	2,096
Common stock warrants	—	132	13	132
Total	500	7,854	6,743	7,854

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The provision for income taxes for the three and nine months ended September 30, 2017 was approximately $43 thousand and $142 thousand, respectively. The provision for/(benefit from) income taxes for the three and nine months ended September 30, 2016 was approximately $(2) thousand and $68 thousand, respectively. The provision for income taxes consisted primarily of foreign income taxes. For the three and nine months ended September 30, 2017 and 2016, the provision for income taxes differed from the statutory amount primarily due to the Company realizing no benefit for current year losses due to maintaining a full valuation allowance against its domestic net deferred tax assets. The realization of tax benefits of deferred tax assets is dependent upon future levels of taxable income, of an appropriate character, in the periods the items are expected to be deductible or taxable. Based on the available objective evidence, the Company does not believe it is more likely than not that the net deferred tax assets will be realizable. Accordingly, the Company has provided a full valuation allowance against the domestic net deferred tax assets as of September 30, 2017 and December 31, 2016. The Company intends to maintain the remaining valuation allowance until sufficient positive evidence exists to support a reversal of, or decrease in, the valuation allowance. During the three and nine months ended September 30, 2017, there were no material changes to the total amount of unrecognized tax benefits.

Commitments and Contingencies	9 Months Ended
Commitments and Contingencies	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Commitments The Company’s principal commitments consist of future payment obligations under capital leases to finance data centers and other computer and networking equipment purchases, debt agreements (see Note 5), operating lease agreements for office space, research and development, and sales and marketing facilities, and agreements with third parties to provide co-location hosting, telecommunication usage and equipment maintenance services. These commitments as of December 31, 2016 are disclosed in the Company's Annual Report on Form 10-K for the year ended December 31, 2016, and did not change materially during the nine months ended September 30, 2017 except for the acquisition of certain additional data center and network equipment and software under multiple capital leases, and certain hosting and telecommunications agreements. As of September 30, 2017, the total minimum future payment commitments under these capital leases added during the nine months ended September 30, 2017 were approximately $7.5 million, of which $0.7 million is due during the remainder of 2017, with the remainder of $6.8 million due over approximately 35 months. The Company entered into various hosting and telecommunications agreements for terms of 11 to 36 months commencing on various dates in the first three quarters of 2017. These agreements require the Company to make monthly payments over the service term in exchange for certain network services. The Company's total minimum future payment commitments under these agreements are $4.3 million. Universal Services Fund Liability During the third quarter of 2012, the Company determined that based on its business activities, it is classified as a telecommunications service provider for regulatory purposes and it should make direct contributions to the federal USF and related funds based on revenues it receives from the resale of interstate and international telecommunications services. In order to comply with the obligation to make direct contributions, the Company made a voluntary self-disclosure to the FCC Enforcement Bureau and registered with the USAC, which is charged by the FCC with administering the USF. The Company filed exemption certificates with its wholesale telecommunications service providers in order to eliminate its obligation to reimburse such wholesale telecommunications service providers for their USF contributions calculated on services sold to the Company. In April 2013, the Company began remitting required contributions on a prospective basis directly to USAC. The Company’s registration with USAC subjects it to assessments for unpaid USF contributions, as well as interest thereon and civil penalties, due to its late registration and past failure to recognize its obligation as a USF contributor and as an international carrier. The Company is required to pay assessments for periods prior to the Company’s registration. As of December 31, 2012, the total past due USF contribution being imposed by USAC and accrued by the Company for the period from 2003 through 2012 was $8.1 million, of which $4.7 million was undisputed and $3.4 million was disputed. The Company subsequently updated its filings and increased the liability related to 2008 through 2012 by $0.5 million, arriving at a new total of $3.9 million in disputed liability. In July 2013, the Company and USAC agreed to a financing arrangement for $4.1 million of the undisputed $4.7 million of the unpaid USF contributions whereby the Company issued to USAC a promissory note payable in the principal amount of the $4.1 million and paid off the remaining undisputed $0.6 million. The Company had fully paid the promissory note as of January 2017. In January 2017, the FCC’s Wireline Competition Bureau ruled in the Company's favor with respect to most of the disputed amount. In September 2017, USAC issued a credit to the Company reflecting the FCC's ruling for the $3.1 million of the $3.9 million in disputed liability. In addition, USAC reversed the interest and penalties related to the disputed liability of $3.1 million. The remaining $0.8 million in dispute involves USAC’s assessment of liability for the period of 2003 through 2007, which was prior to the five year window during which the Company was required to maintain financial records for USF contribution purposes. The Company filed a Request for Review (a form of appeal) of this disputed amount with the FCC's Wireline Competition Bureau in 2013, which remains pending. If the Request for Review is not resolved in the Company’s favor, it is possible that the Company will be held to the back assessments of $0.8 million, which includes interest and penalties on that amount. As of September 30, 2017, the accrued liability on the remaining disputed assessments, including interest and penalties for the period of 2003 through 2007, was $0.8 million offset by $0.7 million in other USF credits. State and Local Taxes and Surcharges In April 2012, the Company commenced collecting and remitting sales taxes on sales of subscription services in all the U.S. states in which it determined it was obligated to do so. During the first quarter of 2015, the Company conducted an updated sales tax review of the taxability of sales of its subscription services. As a result, the Company determined that it may be obligated to collect and remit sales taxes on such sales in four additional states. Based on its best estimate of the probable sales tax liability in those four states relating to its sales of subscription services during the period 2011 through 2014, during the three months ended March 31, 2015, the Company recorded a general and administrative expense of $0.6 million to accrue for such taxes. During 2013, the Company analyzed its activities and determined it may be obligated to collect and remit various state and local taxes and surcharges on its usage-based fees. The Company had not remitted state and local taxes on usage-based fees in any of the periods prior to 2014 and therefore accrued a sales tax liability for this contingency. In January 2014, the Company commenced paying such taxes and surcharges to certain state authorities. In June 2014, the Company commenced collecting state and local taxes or surcharges on usage-based fees from its clients on a current basis and remitting such taxes to the applicable U.S. state taxing authorities. During the three and nine months ended September 30, 2017, the Company made no payments for its contingent sales taxes on either usage-based fees or sales of subscription services. During the three and nine months ended September 30, 2016, the Company remitted $0.5 thousand and $0.1 million, respectively, for its contingent sales taxes on both usage-based fees and sales of subscription services. For the three and nine months ended September 30, 2017, the Company recognized a gain of $0.2 million and $0.5 million, respectively, as an offset to general and administrative expense related to its estimated sales tax liability on both usage-based fees and sales of subscription services in the U.S. and Canada. For the three and nine months ended September 30, 2016, the Company recognized a gain of $0.1 million and $0.3 million, respectively, as an offset to general and administrative expense related to its estimated sales tax liability on both usage-based fees and sales of subscription services in the U.S. and Canada, which was not being collected from its clients. As of September 30, 2017 and December 31, 2016, the Company had total accrued liabilities of $1.6 million and $2.1 million, respectively, for such contingent sales taxes and surcharges that were not being collected from its clients but may be imposed by various taxing authorities, of which $0.5 million and $0.6 million, respectively, were included in current “Sales tax liability” on the condensed consolidated balance sheets, and the remaining were included in non-current “Sales tax liability” on the condensed consolidated balance sheets. The Company’s estimate of the probable loss incurred under this contingency is based on its analysis of the source location of its usage-based fees and the regulations and rules in each tax jurisdiction. Legal Matters The Company is involved in various legal and regulatory matters arising in the normal course of business. In management’s opinion, resolution of these matters is not expected to have a material impact on the Company’s consolidated results of operations, cash flows, or its financial position. However, due to the uncertain nature of legal matters, an unfavorable resolution of a matter could materially affect the Company’s future consolidated results of operations, cash flows or financial position in a particular period. The Company expenses legal fees as incurred. The Company is currently involved in the following lawsuits as a defendant. Melcher Litigation On September 28, 2016, a complaint was filed in the United States District Court for the Southern District of California against Five9, Inc., or Five9, as the successor in interest to Face It, Corp., or Face It, and Lance Fried, a former Five9 employee who was the former Chief Executive Officer of Face It. The action, captioned Melcher, et al. v. Five9, Inc., et al., No. 16-cv-02440, or the Melcher Litigation, was filed as a direct action by Carl Melcher, or Melcher, a purported former stockholder of Face It, and his related investment entity Melcher Family Limited Partnership, or MFLP. In the complaint, the plaintiffs alleged that Face It repurchased the plaintiffs’ stock in September 2013 before Five9 acquired Face It, and that in connection with the repurchase, Fried made material misstatements or omissions to Melcher, by failing to disclose that Face It allegedly was in concurrent discussions about a potential sale of the company to Five9. The complaint alleged violations of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and Rule 10b-5 promulgated thereunder, as well as various claims under state law and common law. The complaint sought to set aside Face It’s September 2013 stock repurchase from the plaintiffs, as well as an unspecified amount of damages and an award of attorney’s fees and costs, in addition to other relief. On November 8, 2016, the court entered an order staying the lawsuit and ordered the parties to proceed to arbitration of the dispute before the American Arbitration Association, or AAA. On November 16, 2016, Melcher and MFLP submitted a Demand for Arbitration to AAA against Five9, asserting claims identical to those alleged in the lawsuit. On March 31, 2017, Five9 reached a settlement with the plaintiffs that fully resolved the plaintiffs’ claims against Five9 and provided for mutual releases between the plaintiffs and Five9 in exchange for a one-time payment by Five9 to the plaintiffs of $1.7 million. As a result of the settlement, the AAA arbitration was concluded, and on July 10, 2017 the plaintiffs filed an amended complaint in the Melcher Litigation solely against Fried, removing Five9 as a defendant. NobelBiz Litigation On August 5, 2011, NobelBiz sent a letter to the Company asserting infringement of a patent related to virtual call centers. On April 3, 2012, NobelBiz filed a patent infringement lawsuit against the Company in the United States District Court for the Eastern District of Texas. The patent asserted in the complaint is different, but related, to the patent asserted in the original letter. The lawsuit, NobelBiz Inc. v. Five9, Inc., Case No. 6:12-cv-00243-LED, alleges that the Company’s local caller ID management service infringes United States Patent No. 8135122, or the ‘122 patent. The ‘122 patent, titled “System and Method for Modifying Communication Information (MCI),” issued on March 13, 2012, and according to the complaint is alleged to relate to “a system for processing a telephone call from a call originator (also referred to as a calling party) to a call target (also referred to as a receiving party), where the system accesses a database storing outgoing telephone numbers, selects a replacement telephone number from the outgoing telephone numbers based on the telephone number of the call target, and originates an outbound call to the call target with a modified outgoing caller identification (‘caller ID’).” NobelBiz seeks damages in the form of lost profits as well as injunctive relief. The lawsuit is one of several lawsuits filed by NobelBiz against various companies including TCN Inc., LiveVox, Inc. and Global Connect LLC. On March 28, 2013, the court granted the Company’s motion to transfer the case to the United States District Court for the Northern District of California. Subsequently, NobelBiz amended its complaint to add claims related to U.S. Patent No. 8565399, or the ‘399 patent, which is a continuation in the same family as the ‘122 patent and addresses the same technology. The Company responded to the complaint and amended complaint by asserting noninfringement and invalidity of the ‘122 and ‘399 patents. On January 16, 2015, the court issued an order regarding claim construction of the two patents-in-suit. On March 7, 2016, the court stayed the case pending an appeal in lawsuits involving NobelBiz, Global Connect and TCN that also involve the ‘122 and ‘399 patents. On July 19, 2017, the Federal Circuit Court of Appeal issued a ruling confirming the claim construction in Five9’s lawsuit and resolving the appeal in favor of Global Connect and TCN and against NobelBiz. Subsequently, the district court set a new schedule for the litigation in light of this ruling with motions for summary judgment set to occur in 2018. The Company has investigated the claims alleged in the complaint and believes that it has good defenses to the claims. While the Company does not believe that it is probable that a loss has been incurred, the ultimate resolution of the matter could potentially result in a loss. Management’s best estimate of the low end of the range of the potential loss is zero. At this time, it is not possible to reasonably estimate the high end of the range of the potential loss, which could be material to the Company’s results of operations. Accordingly, the Company has not accrued a loss related to this matter. Indemnification Agreements In the ordinary course of business, the Company enters into agreements of varying scope and terms pursuant to which it agrees to indemnify clients, vendors, lessors, business partners and other parties with respect to certain matters, including, but not limited to, losses arising out of breach of such agreements, services to be provided by the Company or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with directors and certain officers and employees that will require it, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. Other than as described below, no demands have been made upon the Company to provide indemnification under such agreements and there are no claims that it is aware of that could have a material effect on the consolidated balance sheet, consolidated statement of operations and comprehensive loss, or consolidated statements of cash flows. On October 27, 2016, the Company received notice from Lance Fried, a former officer and director of Face It, of his claim for indemnification by the Company (as successor in interest to Face It), and for advancement of all legal fees and expenses he incurs in connection with the defense of the Melcher Litigation. See "Legal Matters" above. As of May 31, 2017, the Company had advanced Mr. Fried $62 thousand in connection with this claim; however, the Company disputes that Mr. Fried is entitled to advancement in connection with the Melcher Litigation. On July 31, 2017, Mr. Fried filed a complaint against the Company in the Court of Chancery for the State of Delaware, in which he alleges that the Company breached advancement obligations to him. In the lawsuit, Mr. Fried seeks advancement of his legal fees and expenses in connection with the defense of the Melcher Litigation, payment of his legal fees and expenses incurred in connection with his advancement action, and interest. The Company believes the action is without merit and is defending it vigorously. Regardless of the outcome of Mr. Fried’s advancement lawsuit against the Company, Mr. Fried is required to reimburse the Company for any amounts advanced to him if it is ultimately determined that Mr. Fried is not entitled to indemnification in connection with the Melcher Litigation. In addition, the Company believes that it has indemnification rights against the former stockholders of Face It (including Mr. Fried) for all losses that are incurred by the Company in connection with the Melcher Litigation, including without limitation, amounts incurred to indemnify or advance the legal fees and expenses of Mr. Fried pursuant to his indemnification claim against the Company.

Geographical Information

9 Months Ended

Sep. 30, 2017

Segment Reporting [Abstract]

Geographical Information

The following table is a summary of revenues by geographic region based on client billing address and has been estimated based on the amounts billed to clients during the periods presented (in thousands).



	Three Months Ended				Nine Months Ended
	September 30, 2017		September 30, 2016		September 30, 2017		September 30, 2016
United States	$	46,897	$	38,251	$	136,085	$	109,948
International	3,184		2,731		8,737		7,935
Total revenue	$	50,081	$	40,982	$	144,822	$	117,883

The following table summarizes total property and equipment, net in the respective locations (in thousands).



	September 30, 2017		December 31, 2016
United States	$	16,585	$	13,025
International	1,373		1,663
Property and equipment, net	$	17,958	$	14,688

Description of Business and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. Therefore, these condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016. In the opinion of management, the condensed consolidated financial statements reflect all adjustments, which are normal and recurring in nature, necessary for fair financial statement presentation. All intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. The significant estimates made by management affect revenue, the allowance for doubtful accounts, intangible assets, goodwill, loss contingencies, including the Company’s accrual for federal fees and sales tax liability, accrued liabilities, stock-based compensation, provision for income taxes and uncertain tax positions. Management periodically evaluates such estimates and they are adjusted prospectively based upon such periodic evaluation. Actual results could differ from those estimates.
Recently Adopted Accounting Pronouncements and Recent Accounting Pronouncements Not Yet Effective	Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. This ASU simplifies several aspects of the accounting for share-based payment transactions, including the accounting for income taxes, forfeitures, and statutory tax withholding requirements, as well as classification in the statement of cash flows. The guidance was effective for the Company beginning in the first quarter of 2017. Accordingly, commencing in January 2017, the Company accounted for forfeitures as they occurred, rather than by estimating expected forfeitures. The net effect of this change was recognized as a $0.2 million reduction to accumulated deficit in the condensed consolidated financial statements. Upon adoption of the new standard, all excess tax benefits and tax deficiencies (including tax benefits of dividends on share-based payment awards) are recognized as income tax expense or benefit in the income statement. The tax effects of exercised or vested awards are treated as discrete items in the reporting period in which they occur. The Company also recognizes excess tax benefits regardless of whether the benefit reduces taxes payable in the current period. The Company has applied the modified retrospective adoption approach beginning January 1, 2017 and prior periods have not been adjusted. As a result, the Company established a net operating loss deferred tax asset of $5.3 million to account for prior period excess tax benefits through retained earnings, however an offsetting valuation allowance of $5.3 million was also established through retained earnings because it is not more likely than not that the deferred tax asset will be realized due to historical and expected future losses, such that there is no impact on the Company’s condensed consolidated financial statements. Recent Accounting Pronouncements Not Yet Effective In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which requires measurement and recognition of expected credit losses for certain types of financial assets held. ASU 2016-13 is effective for the Company in its first quarter of 2020, and earlier adoption is permitted beginning in the first quarter of 2019. The Company is currently evaluating the impact of ASU 2016-13 on its consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). Under the new guidance, a lessee will be required to recognize assets and liabilities for both finance, or capital, and operating leases with lease terms of more than 12 months. The ASU also will require disclosures to help investors and other financial statement users better understand the amount, timing, and uncertainty of cash flows arising from leases. Lessor accounting will remain largely unchanged from current GAAP. In transition, lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach that includes a number of optional practical expedients that entities may elect to apply. This guidance is effective for the Company beginning in the first quarter of 2019. Early adoption is permitted. The Company is currently assessing the effect the guidance will have on its consolidated financial statements. In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers: Topic 606 and issued subsequent amendments to the initial guidance in August 2015, March 2016, April 2016 and May 2016 within ASU 2015-04, ASU 2016-08, ASU 2016-10 and ASU 2016-12, respectively (ASU 2014-09, ASU 2015-04, ASU 2016-08, ASU 2016-10 and ASU 2016-12 collectively, Topic 606). Topic 606 requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers and will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. Topic 606 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than are required under existing GAAP. Topic 606 is effective for the Company's annual and interim reporting periods beginning January 1, 2018 using either a retrospective or a cumulative effect transition method. The Company has developed an implementation plan to adopt this new guidance based on the cumulative effect transition method. As part of this plan, the Company is currently assessing the impact of the new guidance on its results of operations. Based on procedures performed to date, the Company does not believe the adoption of Topic 606 will have a material impact on the timing of revenue, but the adoption will have a material impact on commission expense. The Company will continue to evaluate this assessment in 2017. The Company intends to adopt Topic 606 on January 1, 2018.

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Schedule of Assets Carried at Fair Value

The fair value of assets carried at fair value was determined using the following inputs (in thousands):



	September 30, 2017
	Total		Level 1		Level 3
Assets
Cash equivalents
Money market funds	$	20,060	$	20,060	$	—
Other Assets
Embedded conversion option held for investment	$	946	$	—	$	946

	December 31, 2016
	Total		Level 1		Level 3
Assets
Cash equivalents
Money market funds	$	20,069	$	20,069	$	—
Other Assets
Embedded conversion option held for investment	$	873	$	—	$	873

Fair Value Measured and Recorded Using Significant Unobservable Inputs

Fair Value Measured and Recorded Using Significant Unobservable Inputs (Level 3) (in thousands):



	September 30, 2017
Beginning balance	$	873
Total gains included in earnings	73
Ending balance	$	946

Financial Statement Components (Tables)

9 Months Ended

Sep. 30, 2017

Financial Statement Components [Abstract]

Schedule of Cash and Cash Equivalents

Cash and cash equivalents consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Cash	$	43,304	$	38,053
Money market funds	20,060		20,069
Cash and cash equivalents	$	63,364	$	58,122

Schedule of Accounts Receivable

Accounts receivable, net consisted of the following (in thousands):



	September 30, 2017			December 31, 2016
Trade accounts receivable	$	16,096		$	12,640
Unbilled trade accounts receivable, net of advance client deposits	1,152			1,253
Allowance for doubtful accounts	(17		)	(12		)
Accounts receivable, net	$	17,231		$	13,881

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Prepaid expenses	$	3,034	$	2,199
Other current assets	1,775		809
Prepaid expenses and other current assets	$	4,809	$	3,008

Schedule of Property and Equipment

Property and equipment, net consisted of the following (in thousands):



	September 30, 2017			December 31, 2016
Computer and network equipment	$	44,517		$	37,664
Computer software	6,383			5,133
Internal-use software development costs	500			475
Furniture and fixtures	1,223			1,130
Leasehold improvements	641			624
Property and equipment	53,264			45,026
Accumulated depreciation and amortization	(35,306		)	(30,338		)
Property and equipment, net	$	17,958		$	14,688

Schedule of Capital Leased Property and Equipment

Property and equipment capitalized under capital lease obligations consist primarily of computer and network equipment and was as follows (in thousands):



	September 30, 2017			December 31, 2016
Gross	$	44,471		$	35,504
Less: accumulated depreciation and amortization	(29,072		)	(23,128		)
Total	$	15,399		$	12,376

Schedule of Accrued and Other Current Liabilities

Accrued and other current liabilities consisted of the following (in thousands):



	September 30, 2017		December 31, 2016
Accrued compensation and benefits	$	9,669	$	7,456
Accrued expenses	2,298		2,148
Accrued and other current liabilities	$	11,967	$	9,604

Intangible Assets (Tables)

9 Months Ended

Sep. 30, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Intangible Assets

The components of intangible assets were as follows (in thousands):



	September 30, 2017							December 31, 2016
	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount		Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Developed technology	$	2,460	$	(1,390	)	$	1,070	$	2,460	$	(1,126	)	$	1,334
Customer relationships	520		(411		)	109		520		(333		)	187
Domain names	50		(39		)	11		50		(32		)	18
Non-compete agreements	140		(140		)	—		140		(140		)	—
Total	$	3,170	$	(1,980	)	$	1,190	$	3,170	$	(1,631	)	$	1,539

Schedule of Expected Future Amortization Expense of Intangible Assets

As of September 30, 2017, the expected future amortization expense for intangible assets was as follows (in thousands):



Period	Expected Future Amortization Expense
2017	$	117
2018	442
2019	351
2020	280
Total	$	1,190

Debt (Tables)

9 Months Ended

Sep. 30, 2017

Debt Disclosure [Abstract]

Schedule of Outstanding Debt

As of September 30, 2017 and December 31, 2016, the Company’s outstanding debt is summarized as follows (in thousands):



	September 30, 2017			December 31, 2016
Promissory note to USAC	$	—		$	120
FCC civil penalty	500			1,000
Total notes payable, gross	500			1,120
Less: discount	(18		)	(79		)
Total notes payable, net carrying value	482			1,041
Revolving line of credit	32,594			32,594
Interest accretion under 2016 line of credit	4			19
Total debt, net carrying value	$	33,080		$	33,654
Less: current portion of debt *	$	(486	)	$	(742	)
Total debt, less current portion **	$	32,594		$	32,912


* Included in ‘Notes payable’ in the condensed consolidated balance sheets.
** Included in ‘Notes payable - less current portion’ and 'Revolving line of credit — less current portion' in the condensed consolidated balance sheets.

Schedule of Maturities of Debt

Maturities of the Company’s outstanding debt as of September 30, 2017 are as follows (in thousands):



Period	Amount to Mature
2017	$	167
2018	333
2019	32,594
Total	$	33,094

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2017

Equity [Abstract]

Schedule of Common Stock Reserved for Future Issuance

Shares of common stock reserved for future issuance related to outstanding equity awards, common stock warrants, and employee equity incentive plans were as follows (in thousands):



		September 30, 2017
Stock options outstanding		4,597
Restricted stock units outstanding		2,133
Shares available for future grant under 2014 Plan		7,526
Shares available for future issuance under ESPP		1,495
Common stock warrants outstanding		13
Total shares of common stock reserved		15,764

Schedule of Stock Option Activity

A summary of the Company’s stock option activity during the nine months ended September 30, 2017 is as follows (in thousands, except years and per share data):



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2016	5,556		$	5.23
Options granted (weighted average grant date fair value of $8.20 per share)	511		16.83
Options exercised	(1,336	)	2.46
Options forfeited or expired	(134	)	12.64
Outstanding as of September 30, 2017	4,597		$	7.10	6.3	$	77,235

Schedule of RSU Activity

A summary of the Company's restricted stock unit ("RSU") activity during the nine months ended September 30, 2017 is as follows (in thousands, except per share data):



	Number of Shares		Weighted Average Grant Date Fair Value Per Share
Outstanding as of December 31, 2016	2,019		$	7.65
RSUs granted	1,022		17.31
RSUs vested and released	(745	)	8.68
RSUs forfeited	(163	)	10.92
Outstanding as of September 30, 2017	2,133		$	11.62

Schedule of Stock-based Compensation Expense

Stock-based compensation expenses for the three and nine months ended September 30, 2017 and 2016 were as follows (in thousands):



	Three Months Ended				Nine Months Ended
	September 30, 2017		September 30, 2016		September 30, 2017		September 30, 2016
Cost of revenue	$	599	$	357	$	1,608	$	951
Research and development	797		547		2,235		1,510
Sales and marketing	1,084		626		3,236		1,604
General and administrative	1,240		989		3,624		2,862
Total stock-based compensation	$	3,720	$	2,519	$	10,703	$	6,927



	Stock Option		RSU		ESPP
Unrecognized stock-based compensation expense	$	6,461	$	23,056	$	181
Weighted-average amortization period	2.8 years		2.9 years		0.1 years

Schedule of Valuation Assumptions, Stock Option

The weighted-average assumptions used to value stock options granted during the three and nine months ended September 30, 2017 and 2016 were as follows:



Stock Options	Three Months Ended		Nine Months Ended
	September 30, 2017	September 30, 2016	September 30, 2017	September 30, 2016
Expected term (years)	6.1	6.1	6.2	6.0
Volatility	53%	48%	49%	48%
Risk-free interest rate	2.0%	1.2%	2.0%	1.5%
Dividend yield	—	—	—	—

Net Income (Loss) Per Share (Tables)

9 Months Ended

Sep. 30, 2017

Earnings Per Share [Abstract]

Basic and Diluted Net Loss Per Share

The following table presents the calculation of basic and diluted net income (loss) per share (in thousands, except per share data).



	Three Months Ended					Nine Months Ended
	September 30, 2017		September 30, 2016			September 30, 2017			September 30, 2016
Net income (loss)	$	924	$	(3,890	)	$	(8,338	)	$	(12,269	)
Weighted-average shares of common stock outstanding	55,310		52,708			54,579			52,078
Basic net income (loss) per share	$	0.02	$	(0.07	)	$	(0.15	)	$	(0.24	)

Net income (loss)	$	924	$	(3,890	)	$	(8,338	)	$	(12,269	)
Weighted-average shares of common stock outstanding	55,310		52,708			54,579			52,078
Effect of dilutive shares	4,131		—			—			—
Weighted average shares of common stock outstanding	59,441		52,708			54,579			52,078
Diluted net income (loss) per share	$	0.02	$	(0.07	)	$	(0.15	)	$	(0.24	)

Schedule of Antidilutive Securities Excluded from Computation of Net Loss Per Share

The following securities were excluded from the calculation of diluted net income (loss) per share because their effect would have been anti-dilutive (in thousands).



	Three Months Ended		Nine Months Ended
	September 30, 2017	September 30, 2016	September 30, 2017	September 30, 2016
Stock options	471	5,626	4,597	5,626
Restricted stock units	29	2,096	2,133	2,096
Common stock warrants	—	132	13	132
Total	500	7,854	6,743	7,854

Geographical Information (Tables)

9 Months Ended

Sep. 30, 2017

Segment Reporting [Abstract]

Schedule of Revenues and Property and Equipment by Geographic Region



	Three Months Ended				Nine Months Ended
	September 30, 2017		September 30, 2016		September 30, 2017		September 30, 2016
United States	$	46,897	$	38,251	$	136,085	$	109,948
International	3,184		2,731		8,737		7,935
Total revenue	$	50,081	$	40,982	$	144,822	$	117,883

The following table summarizes total property and equipment, net in the respective locations (in thousands).



	September 30, 2017		December 31, 2016
United States	$	16,585	$	13,025
International	1,373		1,663
Property and equipment, net	$	17,958	$	14,688

Description of Business and Summary of Significant Accounting Policies (Details) - ASU 2016-09 $ in Millions	Jan. 01, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset	$ 5.3
Deferred tax asset, valuation allowance	5.3
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect on financial statements	$ 0.2

Fair Value Measurements - Schedule of Assets Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Money market funds
Assets
Cash equivalents	$ 20,060	$ 20,069
Money market funds \| Level 1
Assets
Cash equivalents	20,060	20,069
Money market funds \| Level 3
Assets
Cash equivalents	0	0
Embedded conversion option held for investment
Assets
Other Assets	946	873
Embedded conversion option held for investment \| Level 1
Assets
Other Assets	0	0
Embedded conversion option held for investment \| Level 3
Assets
Other Assets	$ 946	$ 873

Fair Value Measurements - Reconciliation of Unobservable Inputs (Details)	9 Months Ended
	Sep. 30, 2017 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 873,000
Total gains included in earnings	73,000
Ending balance	946,000
Transfer in (out) of Level 3 from other levels	0
Assets measured at fair value on nonrecurring basis	0
Liabilities measured at fair value on nonrecurring basis	$ 0

Financial Statement Components - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Financial Statement Components [Abstract]
Cash	$ 43,304	$ 38,053
Money market funds	20,060	20,069
Cash and cash equivalents	$ 63,364	$ 58,122	$ 57,333	$ 58,484

Financial Statement Components - Schedule of Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts Receivable [Line Items]
Allowance for doubtful accounts	$ (17)	$ (12)
Accounts receivable, net	17,231	13,881
Trade accounts receivable
Accounts Receivable [Line Items]
Trade accounts receivable	16,096	12,640
Unbilled trade accounts receivable, net of advance client deposits
Accounts Receivable [Line Items]
Trade accounts receivable	$ 1,152	$ 1,253

Financial Statement Components - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Statement Components [Abstract]
Prepaid expenses	$ 3,034	$ 2,199
Other current assets	1,775	809
Prepaid expenses and other current assets	$ 4,809	$ 3,008

Financial Statement Components - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 53,264	$ 45,026
Accumulated depreciation and amortization	(35,306)	(30,338)
Property and equipment, net	17,958	14,688
Computer and network equipment
Property, Plant and Equipment [Line Items]
Property and equipment	44,517	37,664
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment	6,383	5,133
Internal-use software development costs
Property, Plant and Equipment [Line Items]
Property and equipment	500	475
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment	1,223	1,130
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 641	$ 624

Financial Statement Components - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 1.8	$ 2.0	$ 5.9	$ 5.9
Previously Reported
Property, Plant and Equipment [Line Items]
Estimated useful lives of fixed assets			3 years
Service Life \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful lives of fixed assets			3 years
Service Life \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful lives of fixed assets			5 years
Service Life \| Impact related to change in estimated useful life
Property, Plant and Equipment [Line Items]
Depreciation expense	$ (0.8)		$ (0.8)
Favorable impact to basic and diluted EPS related to change in estimated useful life (in USD per share)	$ 0.02		$ 0.02

Financial Statement Components - Schedule of Capital Leased Property and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Statement Components [Abstract]
Gross	$ 44,471	$ 35,504
Less: accumulated depreciation and amortization	(29,072)	(23,128)
Total	$ 15,399	$ 12,376

Financial Statement Components - Schedule of Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Statement Components [Abstract]
Accrued compensation and benefits	$ 9,669	$ 7,456
Accrued expenses	2,298	2,148
Accrued and other current liabilities	$ 11,967	$ 9,604

Intangible Assets - Schedule of Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets
Gross Carrying Amount	$ 3,170	$ 3,170
Accumulated Amortization	(1,980)	(1,631)
Total	1,190	1,539
Developed technology
Acquired Finite-Lived Intangible Assets
Gross Carrying Amount	2,460	2,460
Accumulated Amortization	(1,390)	(1,126)
Total	1,070	1,334
Customer relationships
Acquired Finite-Lived Intangible Assets
Gross Carrying Amount	520	520
Accumulated Amortization	(411)	(333)
Total	109	187
Domain names
Acquired Finite-Lived Intangible Assets
Gross Carrying Amount	50	50
Accumulated Amortization	(39)	(32)
Total	11	18
Non-compete agreements
Acquired Finite-Lived Intangible Assets
Gross Carrying Amount	140	140
Accumulated Amortization	(140)	(140)
Total	$ 0	$ 0

Intangible Assets - Schedule of Expected Future Amortization Expense of Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
2017	$ 117
2018	442
2019	351
2020	280
Total	$ 1,190	$ 1,539

Intangible Assets - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Amortization expense related to intangible assets	$ 0.1	$ 0.1	$ 0.3	$ 0.4

Debt - 2016 Loan and Security Agreement (Details) - USD ($)		3 Months Ended		9 Months Ended
	Aug. 01, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Line of Credit Facility [Line Items]
Debt, gross		$ 33,094,000		$ 33,094,000
Payments of debt issuance costs				0	$ 206,000
Loss on extinguishment of debt		0	$ 1,026,000	0	$ 1,026,000
Loan and Security Agreement 2016 \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	$ 50,000,000
Credit facility, amount outstanding	$ 32,600,000
Unused line of credit capacity fee	0.25%
First and second anniversary fee for line of credit	$ 31,250
Credit facility, remaining borrowing capacity		17,400,000		17,400,000
Payments of debt issuance costs	$ 206,000
Debt issuance costs, amortization period	3 years
Unrestricted cash required to be deposited with lenders	$ 25,000,000
Line of Credit
Line of Credit Facility [Line Items]
Debt, gross		$ 32,594,000		$ 32,594,000		$ 32,594,000
Loan and Security Agreements 2014 and 2013
Line of Credit Facility [Line Items]
Loss on extinguishment of debt			1,026,000
Prepayment penalties on extinguishment of debt			400,000
Write-off of unamortized debt discounts			400,000
Write-off of unamortized debt issuance costs			$ 206,000
Prime Rate \| Loan and Security Agreement 2016 \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, basis spread	0.50%
Percentage increase in interest rate if adjusted EBITDA is negative at the end of the quarter	0.25%

Debt - 2013 Loan and Security Agreement (Details) - 2013 Loan and Security Agreement - USD ($) $ in Millions	1 Months Ended
	Oct. 31, 2013	Mar. 31, 2013
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity		$ 20.0
Covenant terms, minimum required cash on deposit at lender		$ 7.5
Revolving Credit Facility \| Prime Rate
Debt Instrument [Line Items]
Debt instrument, basis spread		0.50%
Term Loan
Debt Instrument [Line Items]
Credit facility, amount borrowed under agreement	$ 5.0
Term Loan \| Prime Rate
Debt Instrument [Line Items]
Debt instrument, basis spread	1.50%

Debt - 2014 Loan and Security Agreement (Details)	1 Months Ended	9 Months Ended
Debt - 2014 Loan and Security Agreement (Details)	Feb. 28, 2014 USD ($) lender $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Payments of debt issuance costs		$ 0	$ 206,000
Debt, gross		$ 33,094,000
Term Loan \| 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity	$ 30,000,000
Number of lenders \| lender	2
Credit facility, remaining borrowing capacity	$ 10,000,000.0
Credit facility, minimum incremental borrowing amount	$ 1,000,000
Term Loan \| 2014 Loan and Security Agreement \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Minimum variable rate on term loan	10.00%
Debt instrument, basis spread	9.00%
Term Loan \| Tranche One
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity	$ 20,000,000
Payments of debt issuance costs	400,000
Debt, gross	$ 20,000,000
Common Stock \| Term Loan \| 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Warrants issued \| shares	177,865
Warrants outstanding, exercise price (in USD per share) \| $ / shares	$ 10.12
Common Stock \| Term Loan \| Tranche One
Debt Instrument [Line Items]
Warrants vested, shares \| shares	118,577
Warrants vested, fair value	$ 1,000,000
Common Stock Warrant \| Term Loan \| 2014 Loan and Security Agreement
Debt Instrument [Line Items]
Class of warrant or right, vesting period (in years)	10 years
Warrants unvested, shares \| shares	59,288

Debt - Promissory Note/FCC Civil Penalty (Details) $ in Thousands	1 Months Ended
	Jun. 30, 2015 USD ($) payment	Jul. 31, 2013 USD ($) payment	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt, gross			$ 33,094
Total debt, net carrying value			33,080	$ 33,654
Notes Payable
Debt Instrument [Line Items]
Debt, gross			500	1,120
Discount on debt			18	79
Total debt, net carrying value			482	1,041
Notes Payable \| Promissory Note
Debt Instrument [Line Items]
Debt, gross		$ 4,100
Debt instrument, stated interest rate		12.75%
Debt instrument, number of monthly payments \| payment		42
Notes Payable \| FCC Civil Penalty
Debt Instrument [Line Items]
Debt, gross	$ 2,000		500	1,000
Debt instrument, stated interest rate	12.75%
Debt instrument, number of quarterly payments \| payment	12
Discount on debt	$ 300
Total debt, net carrying value	$ 1,700		$ 500	$ 900

Debt - Schedule of Outstanding Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Jun. 30, 2015
Debt Instrument [Line Items]
Debt, gross	$ 33,094
Total debt, net carrying value	33,080	$ 33,654
Less: current portion of debt	(486)	(742)
Total debt, less current portion	32,594	32,912
Notes Payable
Debt Instrument [Line Items]
Debt, gross	500	1,120
Less: discount	(18)	(79)
Total debt, net carrying value	482	1,041
Notes Payable \| Promissory note to USAC
Debt Instrument [Line Items]
Debt, gross	0	120
Total debt, net carrying value		100
Notes Payable \| FCC Civil Penalty
Debt Instrument [Line Items]
Debt, gross	500	1,000	$ 2,000
Less: discount			(300)
Total debt, net carrying value	500	900	$ 1,700
Line of Credit
Debt Instrument [Line Items]
Debt, gross	32,594	32,594
Long-term Debt, Interest Accretion	$ 4	$ 19

Debt - Schedule of Maturities of Debt (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Debt Disclosure [Abstract]
2017	$ 167
2018	333
2019	32,594
Total	$ 33,094

Stockholders' Equity - Capital Structure (Details) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Equity [Abstract]
Common Stock, shares authorized (in shares)	450,000,000	450,000,000
Common Stock, par value (in USD per share)	$ 0.001	$ 0.001
Common Stock, shares issued (in shares)	55,631,423	53,363,013
Common Stock, shares outstanding (in shares)	55,631,423	53,363,013
Preferred Stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred Stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred Stock, shares issued (in shares)	0	0
Preferred Stock, shares outstanding (in shares)	0	0

Stockholders' Equity - Warrants (Details) - $ / shares	9 Months Ended
Stockholders' Equity - Warrants (Details) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Class of Warrant or Right
Warrant expiration date	2023-10
Common Stock
Class of Warrant or Right
Warrants outstanding (in shares)	13,013	131,597
Outstanding warrants exercise price, weighted-average (in USD per share)	$ 5.76	$ 9.80

Stockholders' Equity - Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2017	Dec. 31, 2016	Apr. 03, 2014
Class of Stock
Common stock reserved for future issuance (in shares)	15,764,000
Stock options
Class of Stock
Stock options outstanding (in shares)	4,597,000	5,556,000
Restricted stock units
Class of Stock
Restricted stock units outstanding (in shares)	2,133,000	2,019,000
ESPP
Class of Stock
Common stock reserved for future issuance (in shares)	1,495,000		880,000
Common stock warrants outstanding
Class of Stock
Common stock reserved for future issuance (in shares)	13,000
2014 Plan
Class of Stock
Shares available for future grant under 2014 Plan (in shares)	7,526,000

Stockholders' Equity - Equity Incentive Plans (Details) - 2014 Plan - shares	Jan. 01, 2017	Jan. 01, 2016	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award
Automatic annual increase in common stock reserved for issuance, percentage			5.00%
Automatic annual increase in common stock reserved for issuance, shares	2,668,150	2,558,231
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award
Number of shares originally authorized for issuance			5,300,000

Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Class of Stock
Closing market price of common stock (in USD per share)	$ 23.90
Stock options
Options Outstanding [Roll Forward]
Outstanding, beginning balance (in shares) \| shares	5,556
Options granted (weighted average grant date fair value of $8.20 per share) (in shares) \| shares	511
Options exercised (in shares) \| shares	(1,336)
Options forfeited or expired (in shares) \| shares	(134)
Outstanding, ending balance (in shares) \| shares	4,597
Options Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding, beginning balance, weighted average exercise price (in USD per share)	$ 5.23
Options granted, weighted average exercise price (in USD per share)	16.83
Options exercised, weighted average exercise price (in USD per share)	2.46
Options forfeited or expired, weighted average exercise price (in USD per share)	12.64
Outstanding, ending balance, weighted average exercise price (in USD per share)	7.10
Weighted average grant date fair value, options granted (in USD per share)	$ 8.20
Outstanding, ending balance, weighted average remaining contractual life	6 years 3 months 3 days
Outstanding, ending balance, aggregate intrinsic value \| $	$ 77,235

Stockholders' Equity - RSU Activity (Details) - Restricted stock units shares in Thousands	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Number of Shares
Outstanding, beginning balance (in shares) \| shares	2,019
RSUs granted (in shares) \| shares	1,022
RSUs vested and released (in shares) \| shares	(745)
RSUs forfeited (in shares) \| shares	(163)
Outstanding, ending balance (in shares) \| shares	2,133
Weighted Average Grant Date Fair Value Per Share
Outstanding, beginning balance, weighted-average grant date fair value per share (in USD per share) \| $ / shares	$ 7.65
RSUs granted, weighted average grant date fair value (in USD per share) \| $ / shares	17.31
RSUs vested and released, weighted average grant date fair value (in USD per share) \| $ / shares	8.68
RSUs forfeited, weighted average grant date fair value (in USD per share) \| $ / shares	10.92
Outstanding, ending balance, weighted-average grant date fair value per share (in USD per share) \| $ / shares	$ 11.62

Stockholders' Equity - ESPP (Details) - $ / shares	May 15, 2017	Sep. 30, 2017	Jan. 01, 2017	Jan. 01, 2016	Apr. 03, 2014
Share-based Compensation Arrangement by Share-based Payment Award
Common stock reserved for future issuance (in shares)		15,764,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award
Common stock reserved for future issuance (in shares)		1,495,000			880,000
Automatic annual increase in common stock reserved for issuance, percentage					1.00%
Maximum annual increase of shares reserved					1,000,000
Automatic annual increase in common stock reserved for issuance, shares			533,630	511,646
Purchases made under the ESPP	141,222
Weighted average purchase price (in USD per share)	$ 12.75

Stockholders' Equity - Stock-based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Stock-based Compensation Expense
Stock-based compensation expense	$ 3,720	$ 2,519	$ 10,703	$ 6,927
Cost of revenue
Stock-based Compensation Expense
Stock-based compensation expense	599	357	1,608	951
Research and development
Stock-based Compensation Expense
Stock-based compensation expense	797	547	2,235	1,510
Sales and marketing
Stock-based Compensation Expense
Stock-based compensation expense	1,084	626	3,236	1,604
General and administrative
Stock-based Compensation Expense
Stock-based compensation expense	1,240	$ 989	3,624	$ 2,862
Stock options
Stock-based Compensation Expense
Unrecognized stock-based compensation expense	6,461		$ 6,461
Weighted-average amortization period			2 years 9 months 14 days
Restricted stock units
Stock-based Compensation Expense
Unrecognized stock-based compensation expense	23,056		$ 23,056
Weighted-average amortization period			2 years 11 months 4 days
ESPP
Stock-based Compensation Expense
Unrecognized stock-based compensation expense	$ 181		$ 181
Weighted-average amortization period			1 month 6 days

Stockholders' Equity - Valuation Assumptions (Details) - Stock options	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock
Expected term (years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 2 months 12 days	6 years
Volatility	53.00%	48.00%	49.00%	48.00%
Risk-free interest rate	2.00%	1.20%	2.00%	1.50%
Dividend yield	0.00%	0.00%	0.00%	0.00%

Net Income (Loss) Per Share - Basic and Diluted Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income (loss)	$ 924	$ (3,890)	$ (8,338)	$ (12,269)
Basic (in USD per share)	$ 0.02	$ (0.07)	$ (0.15)	$ (0.24)
Weighted-average shares of common stock outstanding (in shares)	55,310	52,708	54,579	52,078
Effect of dilutive shares (in shares)	4,131	0	0	0
Weighted average shares of common stock outstanding (in shares)	59,441	52,708	54,579	52,078
Diluted (in USD per share)	$ 0.02	$ (0.07)	$ (0.15)	$ (0.24)

Net Income (Loss) Per Share - Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share
Anti-dilutive securities excluded from calculation of diluted net loss per share	500	7,854	6,743	7,854
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share
Anti-dilutive securities excluded from calculation of diluted net loss per share	471	5,626	4,597	5,626
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share
Anti-dilutive securities excluded from calculation of diluted net loss per share	29	2,096	2,133	2,096
Common stock warrants \| Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share
Anti-dilutive securities excluded from calculation of diluted net loss per share	0	132	13	132

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Provision for (benefit from) income taxes	$ 43	$ (2)	$ 142	$ 68

Commitments and Contingencies - Commitments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Capital lease obligations
Other Commitments [Line Items]
Capital lease future payment obligations incurred	$ 7.5
Capital lease future minimum payments, remainder of 2017	0.7
Capital lease future minimum payments, due after 2017	$ 6.8
Period remainder after 2017 of minimum payments is due	35 months
Hosting and telecommunications agreement
Other Commitments [Line Items]
Various agreements total minimum future payment commitments	$ 4.3
Minimum \| Hosting and telecommunications agreement
Other Commitments [Line Items]
Period for various agreements	11 months
Maximum \| Hosting and telecommunications agreement
Other Commitments [Line Items]
Period for various agreements	36 months

Commitments and Contingencies - Universal Services Fund Liability (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 31, 2013	Mar. 31, 2013	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2012
Loss Contingencies [Line Items]
Accrued federal fees			$ 1,146		$ 2,742
Increase (decrease) in accrual federal fees			(2,133)	$ 0
Debt, gross			33,094
Long-term Debt			33,080		$ 33,654
2003 through 2012 \| USF obligations
Loss Contingencies [Line Items]
Accrued federal fees						$ 8,100
2003 through 2012 \| Undisputed portion \| USF obligations
Loss Contingencies [Line Items]
Accrued federal fees						4,700
Payments	$ 600
2003 through 2012 \| Disputed portion \| USF obligations
Loss Contingencies [Line Items]
Accrued federal fees		$ 3,900				$ 3,400
Disputed liability related to the reversal of interest and penalties			3,100
USF credits			3,100
Year 2003 To Year 2007 \| Disputed portion \| USF obligations
Loss Contingencies [Line Items]
Accrued federal fees			800
2008 through 2012 \| Disputed portion \| USF obligations
Loss Contingencies [Line Items]
Increase (decrease) in accrual federal fees		$ 500
Promissory Note \| USF obligations
Loss Contingencies [Line Items]
Debt, gross	$ 4,100
Universal Services Fund
Loss Contingencies [Line Items]
USF credits			$ 700

Commitments and Contingencies - State and Local Taxes and Surcharges (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2015 USD ($) state	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
General and administrative	$ 4,679,000	$ 6,143,000		$ 20,384,000	$ 18,561,000
Sales tax liability	1,207,000			1,207,000		$ 1,476,000
Accrued sales tax liability, current	1,174,000			1,174,000		1,347,000
Contingent sales tax liabilities
Loss Contingencies [Line Items]
Number of states with sales taxes liability \| state			4
Payments	0	500		0	100,000
Sales tax liability	1,600,000			1,600,000		2,100,000
Accrued sales tax liability, current	500,000			500,000		$ 600,000
Out of Period Adjustment \| Contingent sales tax liabilities
Loss Contingencies [Line Items]
General and administrative			$ 600,000
General and administrative expense \| Contingent sales tax liabilities
Loss Contingencies [Line Items]
Sales tax liability gain	$ 200,000	$ 100,000		$ 500,000	$ 300,000

Commitments and Contingencies - Legal Matters (Details) - USD ($)	Mar. 31, 2017	Sep. 30, 2017	May 31, 2017
Indemnification Agreement
Loss Contingencies [Line Items]
Estimate of possible loss			$ 62,000
Melcher Litigation \| Settled Litigation
Loss Contingencies [Line Items]
Litigation settlement, amount awarded to plaintiffs	$ 1,700,000
NobelBiz Inc. v. Five9 \| Pending Litigation \| Minimum \| Patent Infringement
Loss Contingencies [Line Items]
Estimate of possible loss		$ 0

Geographical Information - Schedule of Revenue and Property and Equipment by Geographic Region (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets
Revenue	$ 50,081	$ 40,982	$ 144,822	$ 117,883
Property and equipment, net	17,958		17,958		$ 14,688
United States
Revenues from External Customers and Long-Lived Assets
Revenue	46,897	38,251	136,085	109,948
Property and equipment, net	16,585		16,585		13,025
International
Revenues from External Customers and Long-Lived Assets
Revenue	3,184	$ 2,731	8,737	$ 7,935
Property and equipment, net	$ 1,373		$ 1,373		$ 1,663