Cornerstone OnDemand (CSOD) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 21, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	CSOD
Entity Registrant Name	CORNERSTONE ONDEMAND INC
Entity Central Index Key	1,401,680
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		57,319,023
Entity Public Float			$ 1,125,050,319
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash and cash equivalents	$ 393,576	$ 83,300
Short-term investments	169,551	218,791
Accounts receivable, net	154,428	136,657
Deferred commissions	42,806	36,298
Prepaid expenses and other current assets	21,754	18,467
Total current assets	782,115	493,513
Capitalized software development costs, net	37,431	30,683
Property and equipment, net	20,817	23,962
Long-term investments	96,949	41,046
Intangible assets, net	0	7,421
Goodwill	25,894	25,894
Other assets, net	3,984	1,110
Total Assets	967,190	623,629
Liabilities:
Accounts payable	17,637	24,392
Accrued expenses	57,528	47,619
Deferred revenue, current portion	311,997	272,206
Convertible notes, net	248,025	0
Other liabilities	9,051	2,094
Total current liabilities	644,238	346,311
Convertible notes, net	285,168	238,435
Other liabilities, non-current	1,498	1,794
Deferred revenue, net of current portion	14,166	10,126
Total liabilities	945,070	596,666
Commitments and contingencies (Note 15)
Stockholders’ Equity:
Common stock, $0.0001 par value; 1,000,000 shares authorized, 57,512 and 56,516 shares issued and outstanding at December 31, 2017 and 2016, respectively	6	6
Additional paid-in capital	536,951	476,230
Accumulated deficit	(515,054)	(453,719)
Accumulated other comprehensive income	217	4,446
Total stockholders’ equity	22,120	26,963
Total Liabilities and Stockholders’ Equity	$ 967,190	$ 623,629

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Common stock par value (usd per share)	$ 0.0001	$ 0.0001
Common stock authorized (in shares)	1,000,000,000	1,000,000,000
Common stock issued (in shares)	57,512,000	56,516,000
Common stock outstanding (in shares)	57,512,000	56,516,000

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 481,985	$ 423,124	$ 339,651
Cost of revenue	142,867	135,752	109,864
Gross profit	339,118	287,372	229,787
Operating expenses:
Sales and marketing	240,271	225,631	207,026
Research and development	61,975	46,977	40,991
General and administrative	84,589	70,956	49,877
Restructuring	1,539	0	0
Amortization of certain acquired intangible assets	0	150	600
Total operating expenses	388,374	343,714	298,494
Loss from operations	(49,256)	(56,342)	(68,707)
Other income (expense):
Interest income	2,951	1,702	894
Interest expense	(14,762)	(12,924)	(12,506)
Other, net	1,478	1,934	(4,016)
Other income (expense), net	(10,333)	(9,288)	(15,628)
Loss before income tax provision	(59,589)	(65,630)	(84,335)
Income tax provision	(1,746)	(1,207)	(1,181)
Net loss	$ (61,335)	$ (66,837)	$ (85,516)
Net loss per share — basic and diluted (usd per share)	$ (1.07)	$ (1.20)	$ (1.58)
Weighted average common shares outstanding, basic and diluted (in shares)	57,262	55,595	54,171

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net loss	$ (61,335)	$ (66,837)	$ (85,516)
Other comprehensive income, net of tax:
Foreign currency translation adjustment	(3,795)	3,748	686
Net change in unrealized (losses) gains on investments	(434)	88	(247)
Other comprehensive (loss) income, net of tax	(4,229)	3,836	439
Total comprehensive loss	$ (65,564)	$ (63,001)	$ (85,077)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital (Deficit)	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2014		53,826
Beginning balance at Dec. 31, 2014	$ 35,502	$ 5	$ 336,692	$ (301,366)	$ 171
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon the exercise of options (in shares)		565
Issuance of common stock upon the exercise of options	8,448		8,448
Vesting of restricted stock units (in shares)		200
Shares issued under employee stock purchase plan (in shares)		113
Shares issued under employee stock purchase plan	3,035		3,035
Stock-based compensation	45,914		45,914
Net loss	(85,516)			(85,516)
Other comprehensive income (loss), net of tax	439				439
Ending balance (in shares) at Dec. 31, 2015		54,704
Ending balance at Dec. 31, 2015	7,822	$ 5	394,089	(386,882)	610
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon the exercise of options (in shares)		978
Issuance of common stock upon the exercise of options	18,905	$ 1	18,904
Vesting of restricted stock units (in shares)		699
Shares issued under employee stock purchase plan (in shares)		135
Shares issued under employee stock purchase plan	4,286		4,286
Stock-based compensation	58,951		58,951
Net loss	(66,837)			(66,837)
Other comprehensive income (loss), net of tax	3,836				3,836
Ending balance (in shares) at Dec. 31, 2016		56,516
Ending balance at Dec. 31, 2016	$ 26,963	$ 6	476,230	(453,719)	4,446
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon the exercise of options (in shares)	413	414
Issuance of common stock upon the exercise of options	$ 6,777	$ 0	6,777
Vesting of restricted stock units (in shares)		1,035
Shares issued under employee stock purchase plan (in shares)		182
Shares issued under employee stock purchase plan	$ 5,621		5,621
Repurchase of common stock (in shares)	(635)
Repurchase of common stock	$ (22,599)		(22,599)
Stock-based compensation	70,922		70,922
Net loss	(61,335)			(61,335)
Other comprehensive income (loss), net of tax	(4,229)				(4,229)
Ending balance (in shares) at Dec. 31, 2017		57,512
Ending balance at Dec. 31, 2017	$ 22,120	$ 6	$ 536,951	$ (515,054)	$ 217

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (61,335)	$ (66,837)	$ (85,516)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization	35,377	32,392	27,512
Accretion of debt discount and amortization of debt issuance costs	9,833	9,130	8,691
Purchased investment premium, net of amortization	1,135	240	262
Net foreign currency (gain) loss	(2,461)	(7)	1,584
Stock-based compensation expense	65,924	54,699	43,081
Write-off of capitalized software	1,339	0	0
Deferred income taxes	52	(736)	(105)
Changes in operating assets and liabilities:
Accounts receivable	(14,317)	(38,092)	(21,837)
Deferred commissions	(5,249)	(2,543)	(10,296)
Prepaid expenses and other assets	(2,704)	(3,623)	(2,575)
Accounts payable	(6,820)	5,939	4,444
Accrued expenses	8,530	3,727	14,724
Deferred revenue	35,829	43,379	64,774
Other liabilities	2,377	(2,416)	(947)
Net cash provided by operating activities	67,510	35,252	43,796
Cash flows from investing activities:
Purchases of investments	(323,413)	(210,534)	(220,383)
Maturities of investments	314,418	151,533	138,360
Capital expenditures	(7,100)	(6,228)	(15,633)
Capitalized software costs	(20,571)	(16,409)	(13,283)
Net cash used in provided by investing activities	(36,666)	(81,638)	(110,939)
Cash flows from financing activities:
Proceeds from issuance of convertible notes, net	285,077	0	0
Repayment of debt	0	0	(352)
Principal payments under capital lease obligations	0	(33)	(202)
Proceeds from employee stock plans	12,509	23,548	11,559
Repurchases of common stock	(20,734)	0	0
Net cash provided by financing activities	276,852	23,515	11,005
Effect of exchange rate changes on cash and cash equivalents	2,580	(1,520)	(2,728)
Net increase (decrease) in cash and cash equivalents	310,276	(24,391)	(58,866)
Cash and cash equivalents at beginning of period	83,300	107,691	166,557
Cash and cash equivalents at end of period	393,576	83,300	107,691
Supplemental cash flow information:
Cash paid for interest	3,841	3,796	1,915
Cash paid for income taxes	2,243	2,334	1,520
Proceeds from employee stock plans received in advance of stock issuance	575	489	193
Non-cash investing and financing activities:
Assets acquired under capital leases and other financing arrangements	3,467	0	0
Capitalized assets financed by accounts payable and accrued expenses	1,829	2,080	705
Capitalized stock-based compensation	4,998	4,252	2,833
Deferred debt issuance costs included in accrued expenses	152	0	0
Unsettled share repurchase in other liabilities	$ 1,866	$ 0	$ 0

Organization

Organization	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	ORGANIZATION Company Overview Cornerstone OnDemand, Inc. (“Cornerstone” or the “Company”) was incorporated on May 24, 1999 in the state of Delaware and began its principal operations in November 1999 . The Company is a leading global provider of learning and human capital management software, delivered as Software-as-a-Service (“SaaS”). The Company helps organizations around the globe recruit, train and manage their employees. It is one of the world’s largest cloud computing companies. The Company's human capital management platform combines the world’s leading unified talent management solutions with state-of-the-art analytics and HR administration solutions to enable organizations to manage the entire employee lifecycle. Its focus on continuous learning and development helps organizations to empower employees to realize their potential and drive success. The Company works with clients across all geographies, verticals and market segments. Its Recruiting, Learning, Performance and HR Administration suites help with sourcing, recruiting and onboarding new hires; managing training and development requirements; nurturing knowledge sharing and collaboration among employees; goal setting reviews, competency management and continuous feedback; linking compensation to performance; identifying development plans based on performance gaps; streamlining employee data management, self-service and compliance reporting; and then utilizing state-of-the-art analytics capabilities to make smarter, more-informed decisions using data from across the platform for talent mobility, engagement and development so that HR and leadership can focus on strategic initiatives to help their organization succeed. The Company’s management has determined that the Company operates in one segment as it only reports financial information on an aggregate and consolidated basis to the Company’s chief executive officer, who is the Company’s chief operating decision maker. Office Locations The Company is headquartered in Santa Monica, California and has offices in Amsterdam, Netherlands; Auckland, New Zealand; Bangalore, India; Düsseldorf, Germany; Hong Kong; London, United Kingdom; Madrid, Spain; Mumbai, India; Munich, Germany; New Delhi, India; Paris, France; São Paulo, Brazil; Stockholm, Sweden; Sunnyvale, United States; Sydney, Australia; Tel Aviv, Israel; and Tokyo, Japan.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are presented in accordance with accounting standards generally accepted in the United States of America (“GAAP”), and include the accounts of Cornerstone OnDemand, Inc. and its wholly owned subsidiaries. All significant inter-company transactions and balances have been eliminated in consolidation. Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates. On an on-going basis, management evaluates its estimates, including among others those related to: (i) the realization of tax assets and estimates of tax liabilities and reserves, (ii) the recognition and disclosure of contingent liabilities, (iii) the collectability of accounts receivable, (iv) the evaluation of revenue recognition criteria, including the determination of standalone value and estimates of the selling price of multiple-deliverables in the Company’s revenue arrangements, (v) fair values of investments in marketable securities and strategic investments carried at fair value, (vi) the fair values of acquired assets and assumed liabilities in business combinations, (vii) the useful lives of property and equipment, capitalized software and intangible assets, (viii) impairment of long-lived assets, including goodwill, (ix) the amount and period of amortization of the commission payments to record to expense in proportion to the revenue that is recognized, (x) assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options and (xi) assumptions used in the valuation of various types of performance-based awards. These estimates are based on historical data and experience, as well as various other factors that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. The Company engages third-party valuation specialists to assist with the allocation of the purchase price in business combinations. Such estimates required the selection of appropriate valuation methodologies and models and significant judgment in evaluating ranges of assumptions and financial inputs. Business Combinations The results of businesses acquired in a business combination are included in the Company’s consolidated financial statements from the date of the acquisition. Purchase accounting results in assets and liabilities of an acquired business being recorded at their estimated fair values on the acquisition date. Any excess consideration over the fair value of assets acquired and liabilities assumed is recognized as goodwill. The Company performs valuations of assets acquired and liabilities assumed for an acquisition and allocates the purchase price to its respective net tangible and intangible assets. Determining the fair value of assets acquired and liabilities assumed requires management to use significant judgment and estimates including the selection of valuation methodologies, estimates of future revenue and cash flows, discount rates and selection of comparable companies. The Company engages the assistance of valuation specialists in concluding on fair value measurements in connection with determining fair values of assets acquired and liabilities assumed in a business combination. Transaction costs associated with business combinations are expensed as incurred, and are included in general and administrative expenses in the consolidated statement of operations. There were no transaction costs for the years ended December 31, 2017 , 2016 and 2015. Revenue Recognition The Company derives its revenue from the following sources: • Subscriptions to the Company’s products and other offerings on a recurring basis —Clients pay subscription fees for access to the Company’s enterprise human capital management platform, other products and support on a recurring basis. Fees are based on a number of factors, including the number of products purchased, which may include e-learning content, and the number of users having access to a product. The Company generally recognizes revenue from subscriptions ratably over the term of the agreements. • Professional services and other —The Company offers its clients assistance in implementing its products and optimizing their use. Professional services include application configuration, system integration, business process re-engineering, change management and training services. Services are generally billed on a fixed fee basis and to a lessor degree on a time-and-material basis. These services are generally purchased as part of a subscription arrangement and are typically performed within the first several months of the arrangement. Clients may also purchase professional services at any other time. The Company generally recognizes revenue from fixed fee professional services using the proportional performance method over the period the services are performed and as time is incurred for time-and-material arrangements. The Company recognizes revenue when: (i) persuasive evidence of an arrangement for the sale of the Company’s products or professional services exists, (ii) the products have been made available or delivered, or services have been performed, (iii) the sales price is fixed or determinable and (iv) collectability is reasonably assured. The timing and amount the Company recognizes as revenue is determined based on the facts and circumstances of each client arrangement. Evidence of an arrangement consists of a signed client agreement. The Company considers that delivery of a product has commenced once it provides the client with log-in information to access and use the product. If non-standard acceptance periods or non-standard performance criteria exist, revenue recognition commences upon the satisfaction of the non-standard acceptance or performance criteria, as applicable. Standard acceptance or performance clauses relate to the Company’s products meeting certain perfunctory operating thresholds. Fees are fixed based on stated rates specified in the client agreement. If collectability is not considered reasonably assured, revenue is deferred until the fees are collected. The majority of client arrangements include multiple deliverables, such as subscriptions to the Company’s software products and professional services. The Company therefore recognizes revenue in accordance with the guidance for arrangements with multiple deliverables under Accounting Standards Update (“ASU”) 2009-13 “ Revenue Recognition (Topic 605)—Multiple-Deliverable Revenue Arrangements—a Consensus of the Emerging Issues Task Force ,” or ASU 2009-13. As clients do not have the right to the underlying software code for the products, the Company’s revenue arrangements are outside the scope of software revenue recognition guidance. The Company’s agreements generally do not contain any cancellation or refund provisions other than in the event of the Company’s default. For multiple-deliverable revenue arrangements, the Company first assesses whether each deliverable has value to the client on a standalone basis. The Company has determined that the products have standalone value, because, once access is given to a client, the products are fully functional and do not require any additional development, modification or customization. Professional services have standalone value because third-party service providers, distributors or clients themselves can perform these services without the Company’s involvement. The professional services assist clients with the configuration and integration of the Company’s products. The performance of these services generally does not require highly specialized or skilled individuals and are not essential to the functionality of the products. Based on the standalone value of the deliverables, and since clients do not have a general right of return relative to the included professional services, the Company allocates revenue among the separate deliverables in an arrangement under the relative selling price method using the selling price hierarchy established in ASU 2009-13. This hierarchy requires the selling price of each deliverable in a multiple deliverable arrangement to be based on, in descending order: (i) vendor-specific objective evidence of fair value (“VSOE”), (ii) third-party evidence of fair value (“TPE”) or (iii) management’s best estimate of the selling price (“BESP”). The Company is generally not able to determine VSOE or TPE for its deliverables, because the deliverables are sold separately and within a sufficiently narrow price range only infrequently, and because management has determined that there are no third-party offerings reasonably comparable to the Company’s products. Accordingly, total contract values are allocated to subscriptions to the products and professional services based on BESP. However, the amounts allocated to professional services generally do not exceed the contractually stated values of the professional services, as the revenue for subscriptions to the Company’s products is delivered over a longer period of time and is contingent upon delivery. This can result in higher allocations of the total contract value to subscriptions to the Company’s products over and above the relative selling price allocation based on this contingent revenue limitation. The determination of BESP requires the Company to make significant estimates and judgments. The Company considers numerous factors, including the nature of the deliverables themselves; the geography, market conditions and competitive landscape for the sale; internal costs; and pricing and discounting practices. The Company updates its estimates of BESP on an ongoing basis through internal periodic reviews and as events or circumstances may require. After the contract value is allocated to each deliverable in a multiple deliverable arrangement based on the relative selling price method, revenue is recognized for each deliverable based on the pattern in which the revenue is earned. For subscriptions to the products, revenue is recognized on a straight-line basis over the subscription term, which is typically three years. For professional services, revenue is recognized using the proportional performance method over the period the services are performed. For e-learning content and hosting, revenue is recognized ratably over the period the content is delivered or hosting service is provided. In a limited number of cases, the client’s intended use of a product requires enhancements to its underlying features and functionality. In some of these cases, revenue is recognized as one unit of accounting on a straight-line basis from the point at which the enhancements have been made to the product, through the remaining term of the agreement. In other cases where the enhancement is not required for the client’s intended use, revenue is recognized separately for the enhancement and the product. The enhancement revenue is recognized based on the allocated value on a straight-line basis once the enhancement has been made to the product, through the remaining term of the agreement. For arrangements in which the Company resells third-party e-learning training content to clients or hosts client or third-party e-learning training content provided by the client, revenue is recognized in accordance with accounting guidance as to when to report gross revenue as a principal or report net revenue as an agent. The Company recognizes third-party content revenue at the gross amount invoiced to clients when (i) the Company is the primary obligor, (ii) the Company has latitude to establish the price charged and (iii) the Company bears the credit risk in the transaction. For arrangements involving the sale of third-party content, clients are charged for the content based on pay-per-use or a fixed rate for a specified number of users, and revenue is recognized at the gross amount invoiced as the content is delivered. For arrangements where clients purchase third-party content directly from a third-party vendor, or provide it themselves, and the Company integrates the content into a product, the Company charges a fee per user or fee based on estimated bandwidth. In such cases, the fees are recognized at the net amount charged by the Company for hosting services as the content is delivered. The Company records amounts that have been invoiced to its clients in accounts receivable and in either deferred revenue or revenue depending on whether the revenue recognition criteria described above have been met. Deferred revenue that will be recognized during the succeeding twelve month period from the respective balance sheet date is recorded as current deferred revenue and the remaining portion is recorded as noncurrent. Cost of Revenue Cost of revenue consists primarily of costs related to hosting the Company’s products; personnel and related expenses, including stock-based compensation, and related expenses for network infrastructure, IT support, delivery of contracted professional services and on-going client support staff; payments to external service providers contracted to perform implementation services; depreciation of data centers; amortization of capitalized software costs; amortization of developed technology software license rights; content and licensing fees; and referral fees. In addition, the Company allocates a portion of overhead, such as rent, IT costs, depreciation and amortization and employee benefits costs, to cost of revenue based on headcount. Costs associated with providing professional services are recognized as incurred when the services are performed. Out-of-pocket travel costs related to the delivery of professional services are typically reimbursed by the client and are accounted for as both revenue and expense in the period in which the cost is incurred. Commission Payments The Company defers commissions paid to its sales force and related payroll taxes because these amounts are recoverable from the future revenue due to the non-cancelable client agreements that gave rise to the commissions. Commissions are deferred on the balance sheet and are recognized as sales and marketing expense over the term of the client agreement in proportion to the revenue that is recognized. Commissions are considered direct and incremental costs to client agreements and the Company generally commences payment of commissions within 45 to 75 days after execution of client agreements. During the years ended December 31, 2017 , 2016 and 2015 , the Company deferred $48.2 million , $33.3 million and $42.0 million , respectively, of commissions on the balance sheet. During the years ended December 31, 2017 , 2016 and 2015 , the Company recognized $41.7 million , $33.0 million and $32.3 million in commissions expense to sales and marketing expense, respectively. As of December 31, 2017 and 2016 , deferred commissions on the Company’s consolidated balance sheets totaled $42.8 million and $36.3 million , respectively. Research and Development Research and development expenses consist primarily of personnel and related expenses for the Company’s research and development staff, including salaries, benefits, bonuses and stock-based compensation; the cost of certain third-party service providers; and allocated overhead. Research and development expenses, other than software development costs qualifying for capitalization, are expensed as incurred. The Company’s research and development expenses were $62.0 million in 2017 , $47.0 million in 2016 and $41.0 million in 2015 . Advertising Advertising expenses for 2017 , 2016 and 2015 were $9.0 million , $6.6 million and $5.4 million , respectively, and are expensed as incurred. Stock-Based Compensation The Company accounts for stock-based awards granted to employees and directors by recording compensation expense based on the awards’ estimated fair values. The Company grants stock options and restricted stock units that vest over time based on the continuing employment of the employee, as well as restricted stock units that vest based on meeting certain performance targets. The Company estimates the fair value of its restricted stock units based on the closing price of its common stock as of the date of grant. The Company estimates the fair value of its stock options as of the date of grant using the Black-Scholes option-pricing model. Determining the fair value of stock options under this model requires judgment, including estimating (i) the value per share of our common stock, (ii) volatility, (iii) the term of the awards, (iv) the dividend yield and (v) the risk-free interest rate. The assumptions used in calculating the fair value of stock based awards represent the Company’s best estimates, based on management’s judgment and subjective future expectations. These estimates involve inherent uncertainties. If any of the assumptions used in the model change significantly, stock-based compensation recorded for future awards may differ materially from that recorded for awards granted previously. Beginning in 2017, the Company began estimating expected volatility based solely on its historical volatility as a public company. In previous years, the Company estimated this using the average volatility of similar publicly traded companies as sufficient trading history of the Company’s stock was not available. For purposes of determining the expected term of the awards in the absence of sufficient historical data relating to stock option exercises for the Company, it applies a simplified approach in which the expected term of an award is presumed to be the mid-point between the vesting date and the expiration date of the award. The risk-free interest rate for periods within the expected life of an award, as applicable, is based on the United States Treasury yield curve in effect during the period the award was granted. The estimated dividend yield is zero, as the Company has not declared dividends in the past and does not currently intend to declare dividends in the foreseeable future. The following information represents the weighted average of the assumptions used in the Black-Scholes option-pricing model for stock options granted during each of the last three years: For the Years Ended December 31, 2017 2016 2015 Risk-free interest rate n/a 1.4 % 1.8 % Expected term (in years) n/a 5.8 6.0 Estimated dividend yield n/a — % — % Estimated volatility n/a 48.8 % 41.8 % Once the Company has determined the estimated fair value of its stock-based awards, it recognizes the portion of that value that corresponds to the portion of the award that is ultimately expected to vest, taking estimated forfeitures into account. This amount is recognized as an expense over the vesting period of the award using the straight-line method for awards which contain only service conditions, and using the graded vesting method based upon the probability of the performance condition being met for awards which contain performance conditions. The Company estimates forfeitures based upon its historical experience and for each period, the Company reviews the estimated forfeiture rate and makes changes as factors affecting the forfeiture rate calculations and assumptions change. In addition, the Company has issued performance-based restricted stock units that vest based upon continued service over the vesting term and achievement of certain market conditions and performance goals, and others that vest based upon continued service over the vesting term and achievement of certain market conditions or performance goals, established by the Board of Directors, for a predetermined period. The fair value of the performance-based awards containing a market condition are determined using a Monte-Carlo simulation model that factors in the probability of the award vesting. The Company recognizes the fair value of stock-based compensation for awards which contain market-based conditions using the graded vesting method regardless of whether the market based condition is met. The fair value of the performance-based awards containing only a service and performance condition are determined based upon the closing price of the Company’s common stock on the date of the grant and the Company recognizes the fair value of awards containing a performance condition only if it is probable the performance condition will be met. For all performance-based awards, the fair value is not determined until all of the terms and conditions of the award are established. Due to the full valuation allowance provided on its net deferred tax assets, the Company has not recorded any significant tax benefit attributable to stock-based compensation expense as of December 31, 2017 and 2016 . Capitalized Software Costs The Company capitalizes the costs associated with software developed or obtained for internal use, including costs incurred in connection with the development of its products, when the preliminary project stage is completed, management has decided to make the project a part of its future offering and the software will be used to perform the function intended. These capitalized costs include external direct costs of materials and services consumed in developing or obtaining internal-use software, personnel and related expenses for employees who are directly associated with and who devote time to internal-use software projects and, when material, interest costs incurred during the development. Capitalization of these costs ceases once the project is substantially complete and the software is ready for its intended purpose. Costs incurred for upgrades and enhancements to the products are also capitalized. Post-configuration training and maintenance costs are expensed as incurred. Capitalized software costs are amortized to cost of revenue using the straight-line method over an estimated useful life of the software, which is typically three years , commencing when the software is ready for its intended use. The Company does not transfer ownership of or lease its software to its clients. During the years ended December 31, 2017 , 2016 and 2015 , the Company capitalized $24.3 million , $20.9 million and $16.5 million , respectively, of software development costs to the balance sheet. During the years ended December 31, 2017 , 2016 and 2015 , the Company amortized $17.6 million , $13.2 million and $9.1 million to cost of revenue, respectively. Based on the Company’s capitalized software costs at December 31, 2017 , estimated amortization expense of $19.1 million , $12.5 million , $5.7 million and $0.1 million is expected to be recognized in 2018 , 2019 , 2020 and 2021 , respectively. Comprehensive Loss Comprehensive loss encompasses all changes in equity other than those arising from transactions with stockholders, and consists of net loss, currency translation adjustments and unrealized gains or losses on investments. For the years ended December 31, 2017 , 2016 and 2015 , accumulated other comprehensive income (loss) comprised a cumulative translation adjustment and also included net unrealized gains (losses) on investments. Income Taxes The Company uses the liability method of accounting for income taxes. Under the liability method, deferred taxes are determined based on the temporary differences between the financial statement and tax bases of assets and liabilities, using tax rates expected to be in effect during the years in which the bases differences are expected to reverse. A valuation allowance is recorded when it is more likely than not that some of the deferred tax assets will not be realized. In determining the need for valuation allowances, the Company considers projected future taxable income and the availability of tax planning strategies. The Company has recorded a full valuation allowance to reduce its United States, United Kingdom, New Zealand, Hong Kong and Brazil net deferred tax assets to zero, as it has determined that it is not more likely than not that these deferred tax assets will be realized. The Company has assessed its income tax positions and recorded tax benefits for all years subject to examination, based upon its evaluation of the facts, circumstances and information available at each period end. For those tax positions where the Company has determined there is a greater than 50% likelihood that a tax benefit will be sustained, the Company has recorded the largest amount of tax benefit that may potentially be realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. For those income tax positions where it is determined there is less than 50% likelihood that a tax benefit will be sustained, no tax benefit has been recognized. Cash and Cash Equivalents The Company considers cash and cash equivalents to include short-term, highly liquid investments that are readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value, including investments with original or remaining maturities from the date of purchase of three months or less. At December 31, 2017 and 2016 , cash and cash equivalents consisted of cash balances of $24.7 million and $35.2 million , respectively, money market funds backed by U.S. treasury securities of $358.9 million and $48.1 million , respectively, and certificates of deposit of $10.0 million and no ne, respectively. Investments in Marketable Securities The Company’s available-for-sale investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. If the Company determines that an other-than-temporary decline has occurred for debt securities that the Company does not then currently intend to sell, the Company recognizes the credit loss component of an other-than-temporary impairment in other income (expense) and the remaining portion in other comprehensive income (loss). The credit loss component is identified as the amount of the present value of cash flows not expected to be received over the remaining term of the security, based on cash flow projections. In determining whether an other-than-temporary impairment exists, the Company considers: (i) the length of time and the extent to which the fair value has been less than cost; (ii) the financial condition and near-term prospects of the issuer of the securities; and (iii) the Company’s intent and ability to retain the security for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of interest income or expense. In addition, the Company classifies marketable securities as current or non-current based upon the maturity dates of the securities. At December 31, 2017 and 2016 , the Company had $263.5 million and $257.8 million , respectively, of investments in marketable securities. Strategic Investments Since 2014, the Company has invested in equity securities of multiple privately-held companies. The Company accounted for each of these investment using the cost method of accounting, as we do not have significant influence or a controlling financial interest over these entities. These investments are subject to periodic impairment reviews and are considered to be impaired when a decline in fair value is judged to be other-than-temporary. These investments are included in long-term investments on the Consolidated Balance Sheets. Allowance for Doubtful Accounts The Company bases its allowance for doubtful accounts on its historical collection experience and a review in each period of the status of the then-outstanding accounts receivable. A reconciliation of the beginning and ending amount of allowance for doubtful accounts for the years ended December 31, 2017 , 2016 and 2015 , is as follows (in thousands): 2017 2016 2015 Beginning balance, January 1 $ 3,532 $ 2,578 $ 2,177 Additions and adjustments 7,680 3,165 1,368 Write-offs (3,734 ) (2,211 ) (967 ) Ending balance, December 31 $ 7,478 $ 3,532 $ 2,578 The Company recognized bad debt expense of $1.4 million , $0.8 million and $0.7 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. Property and Equipment, Net Property and equipment are recorded at historical cost, less accumulated depreciation and amortization. Depreciation is computed using the straight-line method based upon the estimated useful lives of the assets, generally two to seven years (See Note 6 ). The Company leases equipment under capital lease arrangements. The assets and liabilities under capital lease are recorded at the lesser of the present value of aggregate future minimum lease payments, including estimated bargain purchase options, or the fair value of the asset under lease. Assets under capital lease are depreciated using the straight-line method over the lesser of the estimated useful life of the asset or the term of the lease. Leasehold improvements are depreciated on a straight-line basis over the shorter of their estimated useful lives or lease terms. Repair and maintenance costs are charged to expense as incurred, while renewals and improvements are capitalized. Impairment of Long Lived Assets The Company evaluates the recoverability of its long-lived assets with finite useful lives, including intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. Such triggering events or changes in circumstances may include: a significant decrease in the market price of a long-lived asset, a significant adverse change in the extent or manner in which a long-lived asset is being used, a significant adverse change in legal factors or in the business climate, the impact of competition or other factors that could affect the value of a long-lived asset, a significant adverse deterioration in the amount of revenue or cash flows expected to be generated from an asset group, an accumulation of costs significantly in excess of the amount originally expected for the acquisition or development of a long-lived asset, current or future operating or cash flow losses that demonstrate continuing losses associated with the use of a long-lived asset, or a current expectation that, more likely than not, a long-lived asset will be sold or otherwise disposed of significantly before the end of its previously estimated useful life. The Company performs impairment testing at the asset group level that represents the lowest level for which identifiable cash flows are largely independent of the cash flows of other assets and liabilities. If events or changes in circumstances indicate that the carrying amount of an asset group may not be recoverable and the expected undiscounted future cash flows attributable to the asset group are less than the carrying amount of the asset group, an impairment loss equal to the excess of the asset’s carrying value over its fair value is recorded. Fair value is determined based upon estimated undiscounted future cash flows. During the year ended December 31, 2017 , the Company determined that previously capitalized software costs were impaired resulting in the write-off of $1.3 million , which was recorded in research and development expense in the accompanying Consolidated Statement of Operations. There were no impairment charges related to identifiable long lived assets in the year ended December 31, 2016 . Intangible Assets Identifiable intangible assets primarily consist of trade names and intellectual property and acquisition-related intangibles, including developed technology, customer relationships, non-compete agreements, patents, trade names and trademarks. The Company determines the appropriate useful life of its intangible assets by performing an analysis of expected cash flows of the acquired assets. Intangible assets are amortized over their estimated useful lives

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	NET LOSS PER SHARE The following table presents the Company's basic and diluted net loss per share (in thousands, except per share amounts): For the Years Ended December 31, 2017 2016 2015 Net loss $ (61,335 ) $ (66,837 ) $ (85,516 ) Weighted-average shares of common stock outstanding 57,262 55,595 54,171 Net loss per share — basic and diluted $ (1.07 ) $ (1.20 ) $ (1.58 ) At December 31, 2017 , 2016 and 2015 , the following potential shares were excluded from the computation of diluted net loss per share because their effect would have been anti-dilutive (in thousands): December 31, 2017 2016 2015 Options to purchase common stock, restricted stock units and performance-based restricted stock units 10,143 10,635 10,860 Shares issuable pursuant to employee stock purchase plan 114 89 77 Convertible notes 11,825 4,682 4,682 Common stock warrants 4,682 4,682 4,682 Total shares excluded from net loss per share 26,764 20,088 20,301 Under the treasury stock method, the convertible notes and common stock warrants will have a dilutive impact on net earnings per share when the average stock price for the period exceeds the respective conversion prices and the Company has net income. The Company also entered into note hedge transactions (“Note Hedges”) in connection with the convertible notes with respect to its common stock to minimize the impact of potential economic dilution upon conversion of the convertible notes. The Note Hedges were outstanding as of December 31, 2017 . Since the beneficial impact of the Note Hedges is anti-dilutive, they are excluded from the calculation of diluted net income (loss) per share. See Note 8 of the Notes to Consolidated Financial Statements.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments	INVESTMENTS Investments in Marketable Securities The Company’s investments in available-for-sale marketable securities are made pursuant to its investment policy, which has established guidelines relative to the diversification of the Company’s investments and their maturities, with the principal objective of capital preservation and maintaining liquidity that is sufficient to meet cash flow requirements. The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2017 (in thousands): December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash Equivalent Investments Money market funds $ 358,859 $ — $ — $ 358,859 $ 358,859 $ — Certificate of deposit 10,000 — — 10,000 10,000 — Corporate bonds 74,868 — (220 ) 74,648 — 74,648 U.S. treasury securities 189,310 — (430 ) 188,880 — 188,880 $ 633,037 $ — $ (650 ) $ 632,387 $ 368,859 $ 263,528 The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2016 (in thousands): December 31, 2016 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash Equivalent Investments Money market funds $ 48,136 $ — $ — $ 48,136 $ 48,136 $ — Corporate bonds 60,725 1 (50 ) 60,676 — 60,676 Agency bonds 28,954 2 (26 ) 28,930 — 28,930 U.S. treasury securities 157,829 17 (160 ) 157,686 — 157,686 Commercial paper 10,473 — — 10,473 — 10,473 $ 306,117 $ 20 $ (236 ) $ 305,901 $ 48,136 $ 257,765 As of December 31, 2017 , the Company’s investment in corporate bonds, agency bonds and U.S. treasury securities had a weighted-average maturity date of approximately nine months . Unrealized gains and losses on investments were not significant, and the Company does not believe the unrealized losses represent other-than-temporary impairments as of December 31, 2017 . No marketable securities held have been in a continuous unrealized loss position for more than 12 months as of December 31, 2017 . Strategic Investments During the year ended December 31, 2017 , the Company made $1.5 million in strategic investments. As of December 31, 2017 , the Company had aggregate strategic investments of $3.0 million . During the year ended December 31, 2016 , the Company made $0.6 million in strategic investments. As of December 31, 2016 , the Company had aggregate strategic investments of $2.1 million . The Company accounted for each of these investments using the cost method of accounting, as the Company does not have significant influence or a controlling financial interest over these entities. These investments are subject to periodic impairment reviews and are considered to be impaired when a decline in fair value is judged to be other-than-temporary. During the year ended December 31, 2017 , the Company recognized $0.6 million of impairment losses recorded in Other, net in the accompanying Consolidated Statement of Operations. During the year ended December 31, 2016 , the Company did no t recognize any impairment losses.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	GOODWILL AND INTANGIBLE ASSETS Finite-lived Intangibles The Company has finite-lived intangible assets, which are amortized over their estimated useful lives on a straight-line basis. The following table presents the gross carrying amount and accumulated amortization of finite-lived intangible assets as of December 31, 2017 and 2016 (in thousands): December 31, 2017 December 31, 2016 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Developed technology $ 29,984 $ (29,984 ) $ — $ 29,984 $ (22,711 ) $ 7,273 Software license rights 1,654 (1,654 ) — 1,654 (1,506 ) 148 Total $ 31,638 $ (31,638 ) $ — $ 31,638 $ (24,217 ) $ 7,421 Total amortization expense from finite-lived intangible assets was $7.4 million , $9.3 million and $10.6 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. The amortization expense recognized was related to developed technology and software license rights and was recorded in cost of revenue except for $0.2 million and $0.6 million for the years ended December 31, 2016 and 2015 , respectively, which was recorded in “Amortization of certain acquired intangible assets” in the accompanying Consolidated Statements of Operations. The Company evaluates the recoverability of its long-lived assets with finite useful lives, including intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. There were no impairment charges related to identifiable intangible assets in the years ended December 31, 2017 , 2016 and 2015 . Goodwill There was no change in the carrying amount of goodwill for the years ended December 31, 2017 and 2016 .

Other Balance Sheet Amounts

Other Balance Sheet Amounts	12 Months Ended
	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Other Balance Sheet Amounts	OTHER BALANCE SHEET AMOUNTS The balance of property and equipment, net is as follows (in thousands): Useful Life December 31, 2017 2016 Computer equipment and software 3 – 5 years $ 38,838 $ 32,926 Furniture and fixtures 7 years 3,855 3,837 Leasehold improvements 2 – 6 years 10,046 9,878 Renovation in progress n/a 58 58 52,797 46,699 Less: accumulated depreciation and amortization (31,980 ) (22,737 ) Total property and equipment, net $ 20,817 $ 23,962 Depreciation expense for the years ended December 31, 2017 , 2016 and 2015 was $10.3 million , $9.9 million , $7.6 million , respectively. The balance of accrued expenses is as follows (in thousands): December 31, 2017 2016 Accrued bonuses $ 13,557 $ 13,371 Accrued commissions 12,401 10,616 Other accrued expenses 31,570 23,632 Total accrued expenses $ 57,528 $ 47,619

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	FAIR VALUE OF FINANCIAL INSTRUMENTS Assets and liabilities measured at fair value on a recurring basis included the following as of December 31, 2017 and 2016 (in thousands): December 31, 2017 December 31, 2016 Fair Value Level 1 Level 2 Level 3 Fair Value Level 1 Level 2 Level 3 Cash equivalents $ 368,859 $ 358,859 $ 10,000 $ — $ 48,136 $ 48,136 $ — $ — Corporate bonds 74,648 — 74,648 — 60,676 — 60,676 — Agency bonds — — — — 28,930 — 28,930 — U.S. treasury securities 188,880 — 188,880 — 157,686 — 157,686 — Commercial paper — — — — 10,473 — 10,473 — $ 632,387 $ 358,859 $ 273,528 $ — $ 305,901 $ 48,136 $ 257,765 $ — At December 31, 2017 and 2016 , cash equivalents of $358.9 million and $48.1 million , respectively, consisted of money market funds with original maturity dates of three months or less backed by U.S. Treasury bills. At December 31, 2017 , cash equivalents of $10.0 million consisted of certificate of deposits with original maturity dates of three months or less. As of December 31, 2017 , corporate bonds, agency bonds, U.S. treasury securities and commercial paper were classified within Level 2 of the fair value hierarchy. The bonds were valued using information obtained from pricing services, which obtained quoted market prices from a variety of industry data providers, security master files from large financial institutions, and other third-party sources. The Company performed supplemental analysis to validate information obtained from its pricing services. As of December 31, 2017 , no adjustments were made to such pricing information. Convertible Notes The Company’s convertible notes, including the 2018 Notes and the 2021 Notes described below, are shown in the accompanying Consolidated Balance Sheets at their original issuance value, net of unamortized discount and debt issuance costs, and are not remeasured to fair value each period. The approximate fair value of the Company’s convertible notes as of December 31, 2017 was $552.0 million . The fair value of the 2018 Notes was estimated on the basis of quoted market prices, which, due to limited trading activity, are considered Level 2 in the fair value hierarchy. The fair value of the 2021 Notes were estimated as the principal amount of the 2021 Notes, due to the lack of trading activity as of December 31, 2017.

Debt and Other Financing Arrang

Debt and Other Financing Arrangements	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt and Other Financing Arrangements	DEBT AND OTHER FINANCING ARRANGEMENTS 2018 Convertible Notes In 2013, the Company issued convertible notes (the “2018 Notes”) raising gross proceeds of $253.0 million . The 2018 Notes are governed by an Indenture, dated June 17, 2013, between the Company and U.S. Bank National Association, as trustee (the “2013 Indenture”). The 2018 Notes mature on July 1, 2018, unless earlier repurchased or converted, and bear interest at a rate of 1.50% per year payable semi-annually in arrears on January 1 and July 1 of each year, commencing January 1, 2014. The 2018 Notes are convertible at an initial conversion rate of 18.5046 shares of the Company’s common stock per $1,000 principal amount of the 2018 Notes, which represents an initial conversion price of approximately $54.04 per share, subject to adjustment for anti-dilutive issuances, voluntary increases in the conversion rate and make-whole adjustments upon a fundamental change. A fundamental change includes a change in control, delisting of the Company’s common stock and a liquidation of the Company. Upon conversion, the Company will deliver cash for the principal amount, and the Company has the right to settle any amounts in excess of the principal in cash or shares. Prior to April 1, 2018, the 2018 Notes are only convertible upon satisfaction of certain conditions as follows: • during any calendar quarter after September 30, 2013, if the last reported sale price of common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; • during the five business day period after any five consecutive trading day period in which the trading price per $1,000 principal amount of the 2018 Notes for each trading day of that five consecutive trading day period was less than 98% of the product of the last reported sale price of common stock and the conversion rate on each such trading day; or • upon the occurrence of specified corporate events as defined in the 2013 Indenture. Holders of the 2018 Notes may convert their 2018 Notes at any time on or after April 1, 2018, until the close of business on the second scheduled trading day immediately preceding the maturity date. The holders of the 2018 Notes may require the Company to repurchase all or a portion of their 2018 Notes at a cash repurchase price equal to 100% of the principal amount of the 2018 Notes being repurchased, plus accrued and unpaid interest, upon a fundamental change and events of default, including non-payment of interest or principal and other obligations under the 2013 Indenture. In accounting for the 2018 Notes at issuance, the Company separated the 2018 Notes into debt and equity components pursuant to the accounting standards for convertible debt instruments that may be fully or partially settled in cash upon conversion. The fair value of the debt component was estimated using an interest rate for nonconvertible debt, with terms similar to the 2018 Notes, excluding the conversion feature. The excess of the principal amount of the 2018 Notes over the fair value of the debt component was recorded as a debt discount and a corresponding increase in additional paid-in capital. The debt discount is accreted to interest expense over the term of the 2018 Notes using the interest method. The amount recorded to additional paid-in capital is not to be remeasured as long as it continues to meet the conditions for equity classification. Upon issuance of the $253.0 million of 2018 Notes, the Company recorded $214.3 million to debt and $38.7 million to additional paid-in capital for the debt discount. The Company incurred transaction costs of approximately $7.3 million related to the issuance of the 2018 Notes. In accounting for these costs, the Company allocated the costs to the debt and equity components in proportion to the allocation of proceeds from the issuance of the 2018 Notes to such components. Transaction costs allocated to the debt component of $6.2 million are deferred and amortized to interest expense over the term of the 2018 Notes. The transaction costs allocated to the equity component of $1.1 million were recorded to additional paid-in capital. 2021 Senior Convertible Notes In December 2017, the Company issued $300.0 million principal amount of 5.75% senior convertible notes (the “2021 Notes”) for a purchase price equal to 98% of the principal amount, raising net proceeds of $294.0 million . The 2021 Notes are governed by an Indenture, dated December 8, 2017 between the Company and U.S. Bank National Association, as trustee (the “2017 Indenture”). The 2021 Notes mature on July 1, 2021, unless earlier repurchased or converted, and bear interest at a rate of 5.75% per year payable semi-annually in arrears on January 1 and July 1 of each year, commencing January 1, 2018. The 2021 Notes are convertible at an initial conversion rate of 23.8095 shares of the Company’s common stock per $1,000 principal amount of the 2021 Notes, which represents an initial conversion price of $42.00 per share, subject to adjustment for anti-dilutive issuances, voluntary increases in the conversion rate and make-whole adjustments upon a fundamental change. A fundamental change includes a change in control, delisting of the Company’s common stock and a liquidation of the Company. Upon conversion, the Company will deliver the applicable number of the Company’s common stock and cash in lieu of any fractional shares. Holders of the 2021 Notes may convert their 2021 Notes at any time prior to the close of business on the scheduled trading day immediately preceding the maturity date, subject to a restricted period through December 2018. The holders of the 2021 Notes may require the Company to repurchase all or a portion of their 2021 Notes at a cash repurchase price equal to 100% of the principal amount of the Notes being repurchased, plus the remaining scheduled interest through and including the maturity date, upon a fundamental change and events of default, including non-payment of interest or principal and other obligations under the 2017 Indenture. The 2021 Notes were issued at a two percent discount and was accounted for as debt upon issuance. The Company recorded $300.0 million of debt and $6.0 million for the debt discount. The debt discount is accreted to interest expense over the term of the 2021 Notes using the interest method. The Company incurred debt issuance costs of $9.1 million that were deferred and will be amortized to interest expense over the term of the 2021 Notes. 2018 Notes and 2021 Notes The net carrying amounts of the liability components of the 2018 and 2021 Notes as of December 31, 2017 and 2016 consists of the following (in thousands): December 31, 2017 December 31, 2016 Principal amount $ 553,000 $ 253,000 Unamortized debt discount (10,190 ) (12,550 ) Net carrying amount before unamortized debt issuance costs 542,810 240,450 Unamortized debt issuance costs (9,617 ) (2,015 ) Net carrying value $ 533,193 $ 238,435 The effective interest rate of the liability component is 5.4% and 6.4% for the 2018 Notes and the 2021 Notes, respectively. The interest rate for the 2018 Notes was based on the interest rates of similar liabilities at the time of issuance that did not have associated convertible features. The following table presents the interest expense recognized related to the 2018 Notes and the 2021 Notes for years ended December 31, 2017 , 2016 and 2015 (in thousands): Years Ended December 31, 2017 2016 2015 Contractual interest expense at 1.50% and 5.75% per annum $ 4,897 $ 3,795 $ 3,795 Amortization of debt issuance costs 1,472 1,263 1,202 Accretion of debt discount 8,360 7,867 7,489 Total $ 14,729 $ 12,925 $ 12,486 Net proceeds of approximately $246.0 million and $285.1 million from the 2018 Notes and the 2021 Notes, respectively, were received after payment of the initial purchasers’ offering expenses. The Company used approximately $49.5 million of the net proceeds of the 2018 Notes offering to pay the cost of the Note Hedges described below, which was partially offset by $23.2 million of the proceeds from the Company’s sale of the Warrants also described below. Note Hedges Concurrent with the 2018 Notes that were issued in 2013, the Company entered into note hedges (the “Note Hedges”) with certain bank counterparties, with respect to its common stock. The Company paid $49.5 million for the Note Hedges. The Note Hedges cover approximately 4.7 million shares of the Company’s common stock at a strike price of $54.04 per share, and are exercisable by the Company upon conversion of the 2018 Notes. The Note Hedges will expire upon the maturity of the 2018 Notes. The Note Hedges are intended to reduce the potential economic dilution upon conversion of the 2018 Notes in the event that the fair value per share of the Company’s common stock at the time of exercise is greater than the conversion price of the 2018 Notes. Warrants Separately and concurrently with the entry by the Company into the Note Hedges in 2013, the Company entered into warrant transactions, whereby it sold warrants to the same bank counterparties as the Note Hedges to acquire up to 4.7 million shares of the Company’s common stock at a strike price of $80.06 per share (the “Warrants”), subject to anti-dilution adjustments. The Company received proceeds of $23.2 million from the sale of the Warrants. The Warrants expire at various dates during 2018 and 2019. If the fair value per share of the Company’s common stock exceeds the strike price of the Warrants, the Warrants will reduce diluted earnings per share to the extent that the calculation does not have an anti-dilutive effect. The amounts paid and received for the Note Hedges and the Warrants have been recorded in additional paid-in capital. The fair value of the Note Hedges and the Warrants are not remeasured through earnings each reporting period.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Stockholders' Equity	STOCKHOLDERS ’ EQUITY Capitalization As of December 31, 2017 , the Company’s authorized stock consists of 1,000,000,000 shares of common stock, par value of $0.0001 per share, and 50,000,000 shares of preferred stock, par value of $0.0001 per share. No shares of preferred stock were issued or outstanding at December 31, 2017 and 2016. Share Repurchase Program In November 2017, the Company’s board of directors authorized a $100.0 million share repurchase program of its common stock. The Company may repurchase its common stock for cash in the open market in accordance with applicable securities laws. The timing and amount of any stock repurchase will depend on share price, corporate and regulatory requirements, economic and market conditions, and other factors. The stock repurchase authorization will expire in November 2019 and shares repurchased will be immediately retired. During the year ended December 31, 2017 , the Company repurchased 0.6 million shares of its common stock at an average cost of $35.55 per share for a total expenditure of $22.6 million . At December 31, 2017 , $77.4 million remained available under the share repurchase program.

Stock-Based Awards

Stock-Based Awards	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Awards	STOCK-BASED AWARDS 1999 and 2009 Plans In November 1999, the Company adopted the 1999 Stock Plan (“1999 Plan”) as amended. In January 2009, the Company adopted the 2009 Plan (“2009 Plan”) as amended. Stock options granted under the 1999 and 2009 Plans may be incentive stock options or non-statutory stock options. At December 31, 2017 , no shares are issuable under the 1999 and 2009 Plans. 2010 Plan In March 2011, upon the completion of the Company’s IPO, the Company adopted the 2010 Plan and determined that it will no longer grant any additional awards under the 1999 Plan and the 2009 Plan. However, the 1999 Plan and the 2009 Plan continue to govern the terms and conditions of the outstanding awards previously granted under each respective plan. Upon the adoption of the 2010 Plan, the maximum aggregate number of shares issuable thereunder was 3,680,480 shares, plus (i) any shares subject to stock options or similar awards granted under the 1999 Plan or 2009 Plan prior to March 16, 2011 that expire or otherwise terminate without having been exercised in full and (ii) shares issued pursuant to awards granted under the 1999 Plan and 2009 Plan that are forfeited to or repurchased by the Company after March 16, 2011, with the maximum number of shares to be added to the 2010 Plan from the 1999 Plan and 2009 Plan equal to 5,614,369 shares of common stock. In addition, the number of shares available for issuance under the 2010 Plan will be annually increased on the first day of each fiscal year beginning with 2012, by an amount equal to the lesser of 5,500,000 shares, 4.5% of the outstanding shares of the Company’s common stock as of the last day of the immediately preceding fiscal year, or such other amount as the Company’s Board of Directors determines. Shares issued pursuant to awards under the 2010 Plan that are repurchased by the Company or that expire or are forfeited, as well as shares used to pay the exercise price of an award or to satisfy the minimum tax withholding obligations related to an award, will become available for future grant or sale under the 2010 Plan. In addition, to the extent that an award is paid out in cash rather than shares, such cash payment will not reduce the number of shares available for issuance under the 2010 Plan. The 2010 Plan permits the grant of incentive stock options to employees and the grant of non-statutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to the Company’s employees, directors and consultants. Under the 2010 Plan, 4,128,903 shares remained available for issuance, at December 31, 2017 . Stock Options The exercise price of stock options granted under the 2010 Plan must equal at least the fair market value of the Company’s common stock on the date of grant. The term of an incentive stock option may not exceed ten years ; provided, however, that an incentive stock option held by a participant who owns more than 10% of the total combined voting power of all classes of the Company’s stock, may not have a term in excess of five years and must have an exercise price of at least 110% of the fair market value of the Company’s common stock on the grant date. Restricted Stock Units The Company may also grant restricted stock units under the 2010 Plan. The fair value of each restricted stock unit granted is equal to the grant date fair market value of the Company’s common stock. The payment of restricted stock units may be in the form of cash, shares, or in a combination thereof, as determined by the Board of Directors. During 2017 , the Company granted 1,937,900 restricted stock units under the 2010 Plan, containing service conditions. Performance Units/Performance Shares The Company may also grant performance units and performance shares under the 2010 Plan. Performance units and performance shares are awards that will result in a payment to a participant only if performance goals for a predetermined period, established by the Board of Directors, are achieved or the awards otherwise vest. The fair value of each performance unit and performance share awarded is equal to the grant date fair value of the Company’s common stock when the performance goals are defined solely by reference to the Company’s own operations. The fair value of each performance unit and performance award that contain performance goals tied to performance of the Company’s common stock is estimated using a Monte-Carlo simulation. The payment of performance units and performance shares may be in the form of cash, shares, or a combination thereof, as determined by the Board of Directors. Employee Stock Purchase Plan Under the Company’s 2010 Employee Stock Purchase Plan (“ESPP”) eligible employees are granted the right to purchase shares at the lower of 85% of the fair value of the stock at the time of grant or 85% of the fair value at the time of exercise. The right to purchase shares is granted twice yearly for six month offering periods in June and December and exercisable on or about the succeeding December and June, respectively, on each year. Under the ESPP, 3,045,181 shares remained available for issuance, at December 31, 2017 . The Company recognized compensation expense related to the ESPP of $1.6 million , $1.4 million and $0.9 million for the years ended December 31, 2017 , 2016 and 2015, respectively. Stock Options The Company has granted stock options, which vest upon meeting service conditions. The following table summarizes the stock option activity which contain only service conditions, under the Company’s 1999, 2009 and 2010 Plans (in thousands, except per share and term information): Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (1) Outstanding, December 31, 2016 6,041 $ 32.01 6.2 $ 74,989 Granted — — Exercised (413 ) 16.38 Forfeited (334 ) 35.71 Outstanding, December 31, 2017 5,294 32.99 5.3 40,122 Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (1) Exercisable at December 31, 2017 4,934 $ 32.60 5.2 $ 39,951 Vested and expected to vest at December 31, 2017 5,280 32.98 5.3 40,113 (1) Based on the Company’s closing stock price of $35.33 on December 31, 2017 and $42.31 on December 31, 2016. The following table summarizes information about stock options, which contain only service conditions, under the Company’s equity incentive plans at December 31, 2017 (in thousands except term information): Options Outstanding at December 31, 2017 Options Exercisable at December 31, 2017 Number of Options Weighted Average Remaining Contractual Term (in years) Number of Options Weighted Average Remaining Contractual Term (in years) Range of Exercise Prices $0.34 to $1.65 92 1.9 92 1.9 $5.93 to $8.88 677 2.9 677 2.9 $12.54 to $15.41 132 3.7 132 3.7 $16.24 to $18.82 305 4.0 305 4.0 $20.85 to $23.94 552 4.4 552 4.4 $27.55 to $31.44 207 5.6 193 5.5 $31.64 to $36.15 835 6.8 633 6.6 $38.03 to $45.76 1,093 6.1 1,036 6.0 $46.20 to $56.05 1,401 6.1 1,314 6.1 5,294 5.3 4,934 5.2 The total intrinsic value of options exercised during the years ended December 31, 2017 , 2016 and 2015 was $9.2 million , $18.2 million and $11.4 million , respectively. The total grant date fair value of stock options vested during the years ended December 31, 2017 , 2016 and 2015 was $15.4 million , $24.3 million and $32.3 million , respectively. The Company recognized compensation expense related to stock options of $14.0 million , $23.0 million and $28.0 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. Unrecognized compensation expense relating to stock options was $5.4 million at December 31, 2017 which is expected to be recognized over a weighted-average period of 1.0 years. The aggregate grant date fair value of stock options granted for the years ended December 31, 2016 and 2015 was $1.8 million and $8.4 million , respectively. Restricted Stock Units Restricted stock unit activity for the year ended December 31, 2017 under the Company’s equity incentive plans is summarized as follows (shares in thousands): Number of Shares Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 3,254 $ 36.35 Granted 1,938 37.99 Forfeited (367 ) 36.04 Vested (1,035 ) 36.34 Outstanding at December 31, 2017 3,790 $ 37.22 The Company recognized compensation expense related to restricted stock units of $44.1 million , $31.9 million and $15.0 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. Unrecognized compensation expense related to unissued shares of the Company’s common stock subject to unvested restricted stock units was $105.7 million at December 31, 2017 , which is expected to be recognized as expense over the weighted-average period of 2.7 years. Performance-Based Restricted Stock Units In July 2014, the Compensation Committee of the Company’s Board of Directors (the “Compensation Committee”) approved the issuance of performance-based restricted stock units to an executive officer of the Company. The number of shares of the Company’s common stock issuable upon the vesting of this performance-based restricted stock award is based upon (a) the performance of the Company’s stock price relative to a certain independent market index and (b) the recipient continuing to provide service through the end of the three -year term of the award. Achievement of the target performance level would result in the issuance of 40,600 shares and achievement at the maximum performance level would result in the issuance of 60,900 shares. The Company used a Monte Carlo simulation to estimate the fair value of this award which factors in the probability of the award vesting. The grant date fair value of the award was $1.8 million , which was recognized ratably over the three -year term of the award. In July 2017, based on the performance of the Company’s stock price relative to a certain independent market index, the Company determined it had not achieved the required performance level, which resulted in none of the shares being issued. In December 2014, the Compensation Committee approved the issuance of a special award of performance-based restricted stock units to certain executives of the Company. The number of shares of the Company’s common stock issuable upon the vesting of these performance-based restricted stock unit awards is based upon (a) the performance of the Company’s stock price relative to a certain independent market index, (b) the achievement of the Company’s revenue guidance for each of fiscal year 2015 and 2016 and (c) the recipient continuing to provide services to the Company through the end of the three -year term of the award. The Company finalizes its revenue guidance in February of each year, thus a grant date was established in February 2015 and February 2016 for each of the two tranches of the award related to that year’s revenue guidance. Each tranche is treated as a separate grant and recognized from the date the revenue guidance is determined over the remaining portion of the original three -year term of the award. Achievement of the target performance level would result in the issuance of 535,000 shares and achievement at the maximum performance level would result in the issuance of an aggregate of 1,070,000 shares. The Company used a Monte Carlo simulation to estimate the fair value of each tranche of the awards for which a grant date has been established. The valuation factors in the probability of achieving the performance of the Company’s stock price relative to the market index. In the first quarter of 2015, the aggregate grant date fair value of the first half of the above awards was $9.9 million . As of December 31, 2017 , the aggregate fair value of the first half of the awards was $4.8 million based on the Company’s performance in relation to the revenue guidance for fiscal year 2015. In the first quarter of 2016, the aggregate grant date fair value of the second half of the awards was $3.6 million . In the first quarter of 2017, the Compensation Committee provided clarification on the use of constant currency adjustments to the Company’s performance in relation to the revenue guidance for fiscal year 2016. As a result, during the year ended December 31, 2017 , the Company determined that the aggregate fair value of the awards was $9.8 million , which was recognized over the remaining vesting period of the awards. In December 2017, based on the performance of the Company’s stock price relative to a certain independent market index, the Company determined it had not achieved the required performance level, which resulted in none of the shares being issued. Beginning in 2016, the Compensation Committee designed an annual equity compensation structure to further align the compensation levels of certain executives to the performance of the Company through the issuance of performance-based restricted stock units. The number of shares of the Company’s common stock issuable upon the vesting of these performance-based restricted stock unit awards is based upon the Company meeting composite revenue and cash flow growth targets determined at the time of the grant. The total amount of compensation expense recognized is based on the number of shares that the Company determines are probable of vesting. The estimate will be made each reporting period and determined by the Company’s actual and projected revenue and cash flow performance and the compensation expense will be recognized over the vesting term of the awards. The following table summarizes the Company’s issuances of awards under the new compensation award structure: Grant Date Performance Measures Vesting Term Performance Period # of Shares at Target # of Shares at Maximum Grant Date Fair Value per share July 2016 (a) the Company meeting certain revenue and cash flow targets through December 31, 2018 and (b) the recipient continuing to provide services to the Company through the end of June 2019 Three years Fiscal years 2016, 2017 and 2018 166,600 499,800 $ 38.67 March 2017 (a) the Company meeting certain revenue and cash flow targets through December 31, 2019 and (b) the recipient continuing to provide services to the Company through the end of March 2020 Three years Fiscal years 2017, 2018 and 2019 185,270 555,810 $ 41.73 The Company recognized compensation expense related to all performance-based awards in the aggregate amount of $11.2 million , $2.6 million and $1.9 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. There was no unrecognized compensation expense related to unvested performance-based restricted stock units at December 31, 2017 . Stock-Based Compensation Stock-based compensation expense related to stock options, restricted stock units, the ESPP and performance-based restricted stock units is included in the following line items in the accompanying Consolidated Statements of Operations for the years ended December 31, 2017 , 2016 and 2015 (in thousands): Years Ended December 31, 2017 2016 2015 Cost of revenue $ 4,904 $ 4,732 $ 3,887 Sales and marketing 28,427 25,642 23,604 Research and development 9,630 7,586 6,010 General and administrative 22,869 16,739 9,580 Total $ 65,830 $ 54,699 $ 43,081 In certain instances the Company is responsible for payroll taxes related to stock options exercised or the underlying shares sold by its employees. The Company accrues its obligations at the time of the exercise of the stock options or the sale of the underlying shares.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES On December 22, 2017, the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law making significant changes to the Internal Revenue Code of 1986, as amended. Changes include, but are not limited to, a corporate tax rate decrease from 34% to 21% effective for tax years beginning after December 31, 2017, further limitation on deductibility of interest expense, the transition of U.S international taxation from a worldwide tax system to a territorial system, and a one-time transition tax on the mandatory deemed repatriation of cumulative foreign earnings as of December 31, 2017. Consistent with the guidance under ASC 740, we will record any impacts from enactment of the Tax Act in the fourth quarter of 2017 subject to Staff Accounting Bulletin (“SAB”) 118 which provides for a measurement period to complete the accounting for certain elements of the tax reform. We have calculated our best estimate of the impact of the Tax Act in our year end income tax provision in accordance with our understanding of the Tax Act and guidance available as of the date of this filing. The Tax Act did not result in any material tax expense during fourth quarter of 2017, the period in which the legislation was enacted. The Company remeasured the net deferred tax assets and corresponding valuation allowance at the 21% rate. Further, the impact of the one-time transition tax on the mandatory deemed repatriation of foreign earnings was immaterial due to an aggregate foreign earnings deficit. The changes included in the Tax Act are broad and complex. The final transition impacts of the Tax Act may differ from the above estimate, possibly materially, due to, among other things, changes in interpretations of the Tax Act, any legislative action to address questions that arise because of the Tax Act, any changes in accounting standards for income taxes or related interpretations in response to the Tax Act, or any updates or changes to estimates the company has utilized to calculate the transition impacts, including impacts from changes to current year earnings estimates and foreign exchange rates of foreign subsidiaries. The Securities Exchange Commission has issued rules that would allow for a measurement period of up to one year after the enactment date of the Tax Act to finalize the recording of the related tax impacts. The components of the Company’s loss before provision (benefit) for income taxes are as follows (in thousands): Years Ended December 31, 2017 2016 2015 United States $ (32,853 ) $ (39,107 ) $ (59,797 ) Foreign (26,736 ) (26,523 ) (24,538 ) Loss before provision for income taxes $ (59,589 ) $ (65,630 ) $ (84,335 ) The components of the provision (benefit) for income taxes attributable to continuing operations are as follows (in thousands): Years Ended December 31, 2017 2016 2015 Current income tax provision: Federal $ — $ — $ — State 114 105 147 Foreign 1,580 1,838 1,139 Total current income tax provision 1,694 1,943 1,286 Deferred income tax benefit: Federal — — — State — — — Foreign 52 (736 ) (105 ) Total deferred income tax benefit 52 (736 ) (105 ) Total income tax provision (benefit) $ 1,746 $ 1,207 $ 1,181 On a consolidated basis, the Company has incurred operating losses and has recorded a full valuation allowance against its United States, United Kingdom, New Zealand, Hong Kong and Brazil deferred tax assets for all periods to date and, accordingly, has not recorded a provision (benefit) for income taxes for any of the periods presented other than a provision (benefit) for certain foreign and state income taxes. Certain foreign subsidiaries and branches of the Company provide intercompany services and are compensated on a cost-plus basis, and therefore, have incurred liabilities for foreign income taxes in their respective jurisdictions. The differences in the total provision for income taxes that would result from applying the 34% federal statutory rate to loss before provision for income taxes and the reported provision for income taxes are as follows (in thousands): Years Ended December 31, 2017 2016 2015 U.S. Federal tax benefit at statutory rates $ (20,260 ) $ (22,310 ) $ (28,681 ) State income taxes, net of federal tax benefit (806 ) (855 ) (1,632 ) Foreign rate differential 5,220 3,711 3,964 Stock based compensation 3,182 4,467 4,673 Other permanent differences (494 ) (750 ) (99 ) Deferred adjustments / U.S. rate change 7,811 — — Other 262 1,494 536 Valuation allowance 6,831 15,450 22,420 Total income tax (benefit) provision $ 1,746 $ 1,207 $ 1,181 Major components of the Company’s deferred tax assets (liabilities) at December 31, 2017 and 2016 are as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Accrued expenses $ 2,371 $ 3,037 Long-lived intangible assets and fixed assets — basis difference 19,884 22,146 Net operating loss carryforwards 80,615 68,356 Stock-based compensation 16,886 19,515 Deferred revenue 2,739 4,060 Convertible note hedge 1,467 6,387 Other 2,721 2,276 Total deferred tax assets 126,683 125,777 Valuation allowance (118,606 ) (111,775 ) Deferred tax assets, net of valuation allowance 8,077 14,002 Deferred tax liabilities: Prepaid expenses and deferred commissions (5,672 ) (7,718 ) Convertible note discount (1,074 ) (4,721 ) Other (410 ) (591 ) Total deferred tax liabilities (7,156 ) (13,030 ) Net deferred tax assets (liabilities) $ 921 $ 972 At December 31, 2017 , the Company had federal, state and foreign net operating losses of approximately $250.7 million , $257.9 million and $86.0 million , respectively. The federal net operating loss carryforward will begin expiring in 2022, the state net operating loss carryforward began expiring in 2017, and the foreign net operating loss has an unlimited carryforward period. The Internal Revenue Code of 1986, as amended, imposes substantial restrictions on the utilization of net operating losses in the event of an “ownership change” of a corporation. Accordingly, a company’s ability to use net operating losses may be limited as prescribed under Internal Revenue Code Section 382 (“IRC Section 382”). Events which may cause limitations in the amount of the net operating losses that the Company may use in any one year include, but are not limited to, a cumulative ownership change of more than 50% over a three-year period. Due to the effects of historical equity issuances, the Company has determined that the future utilization of a portion of its net operating losses is limited annually pursuant to IRC Section 382. The Company has determined that none of its net operating losses will expire because of the annual limitation. The Company has recorded a full valuation allowance against its otherwise recognizable United States, United Kingdom, New Zealand, Hong Kong and Brazil deferred income tax assets as of December 31, 2017 . Management has determined, after evaluating all positive and negative historical and prospective evidence, that it is more likely than not that these assets will not be realized. The net increase to the valuation allowance of $6.8 million , $15.5 million and $22.4 million for the years ended December 31, 2017 , 2016 and 2015 , respectively, was primarily due to additional net operating losses generated by the Company. We adopted ASU No. 2016-09 effective January 1, 2017. The ASU eliminates the requirement to delay the recognition of excess tax benefits until they reduce current taxes payable. However, as of January 1, 2017, the previously unrecognized excess tax benefits of $39.4 million had no impact on our accumulated deficit balance as the related U.S. deferred tax assets were fully offset by a valuation allowance. The adoption did not have any other material impacts on the Company’s provision for income taxes. Deferred income taxes have not been provided on the undistributed earnings of the Company’s foreign subsidiaries because the Company’s practice and intent is to permanently reinvest these earnings. The cumulative amount of such undistributed earnings was $3.3 million and $4.0 million at December 31, 2017 and December 31, 2016 , respectively. Any future distribution of these non-U.S. earnings may subject the Company to state income taxes, as adjusted for tax credits, and foreign withholding taxes that the Company estimates would be $0.1 million and $0.8 million at December 31, 2017 and 2016 , respectively. We are also estimating that the transition tax on any undistributed earnings that existed as of December 31, 2017 will be zero . A reconciliation of the beginning and ending amount of unrecognized tax benefits for the years ended December 31, 2017 , 2016 and 2015 is as follows (in thousands): Years Ended December 31, 2017 2016 2015 Balance at January 1 $ 276 $ 276 $ 276 Additions for tax positions related to the current year 995 — — Balance at December 31 $ 1,271 $ 276 $ 276 The provision for uncertain tax positions relates to business in territories outside of the United States. The Company’s policy is to classify interest and penalties on uncertain tax positions as a component of tax expense. An insignificant amount of interest and penalties on unrecognized tax benefits were accrued during the 2017 tax year. The amount of accrued interest and penalties on unrecognized tax benefits was insignificant, as of December 31, 2017 and 2016 . The Company does not expect the change in uncertain tax positions to have a material impact on its financial position, results of operations or liquidity. The recognition of previously unrecognized tax benefits on uncertain tax positions would result in a $1.3 million tax benefit. The Company believes it is reasonably possible that within the next twelve months we may resolve certain matters related to the years under examination, which may result in reductions of our unrecognized tax benefits and income tax expense of up to $1.1 million . The Company is subject to United States federal income tax as well as to income tax in multiple state and foreign jurisdictions, including the United Kingdom. Federal income tax returns of the Company are subject to IRS examination for the 2014 through 2017 tax years. State income tax returns are subject to examination for the 2013 through 2017 tax years. Foreign income tax returns are subject to examination for the 2007 through 2017 tax years.

Restructuring Costs

Restructuring Costs	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Restructuring Costs	RESTRUCTURING COSTS In December 2017, as part of the Company’s new strategic plan to accelerate revenue growth and increase operating margins, the Company approved a restructuring plan to reduce the headcount of the Company’s global service delivery team, as well as the headcount of some of its sales teams, representing a total workforce reduction of approximately six percent. In December 2017, the Company completed the sales team headcount reductions. The Company expects the service delivery headcount reductions to be primarily completed by March 2018. The restructuring is part of the Company’s renewed focus on recurring, or subscription-based, revenue growth and driving cost reductions to accelerate the growth of its operating margins and free cash flow. During the year ended December 31, 2017 , the Company recognized $1.5 million of restructuring costs, which was recorded in “Restructuring” in the accompanying Consolidated Statements of Operations. The restructuring costs consisted primarily of payroll-related costs, such as severance, outplacement costs and continuing healthcare coverage, associated with employee terminations. The Company expects to incur an additional cost of approximately $3.0 million in connection with the restructuring plan in the first quarter of 2018.

Segment and Geographic Informat

Segment and Geographic Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment and Geographic Information	SEGMENT AND GEOGRAPHIC INFORMATION The Company’s management has determined that the Company operates in one segment as it only reports financial information on an aggregate and consolidated basis to its chief executive officer, who is the chief operating decision maker. The Company presents its entity-wide information in the tables below. The following table sets forth the Company’s sources of revenue (dollars in thousands): At or For Year Ended December 31, 2017 2016 2015 Subscription revenue $ 396,764 $ 339,756 $ 270,093 Percentage of subscription revenue to total revenue 82.3 % 80.3 % 79.5 % Professional services revenue $ 85,221 $ 83,368 $ 69,558 Percentage of professional services revenue to total revenue 17.7 % 19.7 % 20.5 % $ 481,985 $ 423,124 $ 339,651 Revenue by geographic region, which is generally based on the address of the Company’s clients as defined in their master subscription agreements, is set forth below (in thousands): Years Ended December 31, 2017 2016 2015 Revenue United States $ 313,729 $ 284,657 $ 228,724 United Kingdom 25,701 27,571 30,104 All other countries 142,555 110,896 80,823 Total revenue $ 481,985 $ 423,124 $ 339,651 Property and equipment by region is set forth below (in thousands): December 31, 2017 2016 Property and equipment, net United States $ 16,468 $ 19,843 United Kingdom 3,378 2,985 All other countries 971 1,134 Total property and equipment, net $ 20,817 $ 23,962

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
401(k) Savings Plan	401(K) SAVINGS PLAN The Company has a defined contribution savings plan (the “Plan”) under Section 401(k) of the Internal Revenue Code. The Plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Company contributions to the Plan may be made at the discretion of the Board of Directors. The Plan provides for a Company matching contribution in an amount equal to 50% of an employee’s contributions up to $2,400 per year, which vests fully after the four th year of employment. The Company incurred approximately $2.0 million , $1.9 million and $1.6 million of matching contribution expenses related to the Plan during the years ended December 31, 2017 , 2016 and 2015, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Leases The Company has various non-cancelable operating leases for its offices and its managed hosting facilities and services. These leases expire at various times through 2021. Certain lease agreements contain renewal options, rent abatement and escalation clauses. The Company recognizes rent expense on a straight-line basis over the lease term, commencing when the Company takes possession of the property. Certain of the Company’s office leases entitle the Company to receive a tenant allowance from the landlord. The Company records tenant allowances as a deferred rent credit, which the Company amortizes on a straight-line basis, as a reduction of rent expense, over the term of the underlying lease. Total rent expense under operating leases was approximately $8.0 million , $7.8 million and $6.7 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. Future minimum lease payments under non-cancelable operating leases at December 31, 2017 are as follows (in thousands): Operating Leases 2018 $ 8,246 2019 826 2020 84 2021 73 2022 — Total minimum lease payments $ 9,229 Letters of Credit During 2015, the Company amended a standby letter of credit in association with its building lease. In addition, the Company maintains standby letters of credit in association with other contractual arrangements. Total letters of credit outstanding at December 31, 2017 was $1.4 million . Other Commitments As of December 31, 2017 , the Company had agreements with various third-party service providers whereby the Company has committed to assign certain dollar amounts or hours of professional service projects related to implementation and other services for clients of the Company’s human capital management platform. In aggregate, these estimated commitments total approximately $12.0 million in 2018, $6.5 million in 2019 and $0.7 million in 2020. As of December 31, 2017 , the Company had software subscription agreements with various service providers with obligations of approximately $8.4 million in 2018, $5.3 million in 2019 and $0.5 million in 2020. As of December 31, 2017 , the Company had a sponsorship agreement with a professional sports franchise with obligations of approximately $0.7 million in 2018 and $0.7 million in 2019. Guarantees and Indemnifications The Company has made guarantees and indemnities under which it may be required to make payments to a guaranteed or indemnified party, in relation to certain transactions, including revenue transactions in the ordinary course of business. The Company is obligated to indemnify its directors and officers to the maximum extent permitted under the laws of the State of Delaware. However, the Company has a directors and officers insurance policy that may reduce its exposure in certain circumstances and may enable it to recover a portion of future amounts that may be payable, if any. The duration of the guarantees and indemnities varies and, in many cases, is indefinite but subject to statutes of limitations. To date, the Company has made no payments related to these guarantees and indemnities. The Company estimates the fair value of its indemnification obligations as insignificant based on this history and the Company’s insurance coverage and therefore has not recorded any liability for these guarantees and indemnities in the accompanying consolidated balance sheets. Litigation During 2017, a patent infringement claim was filed against the Company. This claim has subsequently been dismissed and did not have a material adverse effect on the Company’s business, operating results, cash flows or financial condition. The Company is subject to various legal proceedings and claims that arise in the ordinary course of business. If the Company determines that it is probable that a loss has been incurred and the amount is reasonably estimable, the Company will record a liability. The Company has determined that it does not have a potential liability related to any legal proceedings or claims that would individually or in the aggregate materially adversely affect its financial condition or operating results. Taxes From time to time, various federal, state and other jurisdictional tax authorities undertake review of the Company and its filings. In evaluating the exposure associated with various tax filing positions, the Company accrues charges for possible exposures. The Company believes any adjustments that may ultimately be required as a result of any of these reviews will not be material to its consolidated financial statements.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	RELATED PARTY TRANSACTIONS The Cornerstone OnDemand Foundation (the “Foundation”) empowers communities in the United States and internationally by increasing the impact of the non-profit sector through the utilization of human capital management technology including the Company’s products. The Company’s chief executive officer is on the board of directors of the Foundation. The Company does not direct the Foundation’s activities, and accordingly, the Company does not consolidate the Foundation’s statement of activities with its financial results. During the years ended December 31, 2017 , 2016 and 2015 , the Company provided at no charge certain resources to the Foundation, with approximate values of $3.4 million , $3.3 million and $2.9 million , respectively. An executive of the Company has an ownership interest in The Corner restaurant (the “Corner”). The Company does not direct the Corner’s activities, and accordingly, the Company does not consolidate the Corner’s statement of activities with its financial results. During the year ended December 31, 2017 , the Company recorded $0.3 million in expenses related to the use of the restaurant. During June 2010, an executive officer of an accounting software company joined the Company’s board of directors and resigned in September 2016. For the years ended December 31, 2016 and 2015 , the Company recorded $0.7 million and $0.6 million , respectively, in expenses related to the use of the accounting software from the company whose executive officer served on the Company’s Board of Directors during those years. During May 2017, an executive officer of a cyber security company joined the Company’s board of directors. For the year ended December 31, 2017 , the Company recorded $0.7 million , in expenses related to the products and services provided by the cyber security company. Additionally, for the year ended December 31, 2017 , the Company recognized revenue of $0.3 million related to the cyber security company’s subscription to our products.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS During January 2018, shares issuable under the Company’s 2010 Employee Stock Purchase Plan increased by 575,119 shares and shares issuable under the Company’s 2010 Plan increased by 2,588,036 shares in accordance with the automatic annual increase provisions of such plans. During January and February 2018, the Company entered into software subscription agreements with various service providers with obligations of approximately $3.1 million in 2018, $3.1 million in 2019 and $3.1 million in 2020. During January and February 2018, the Compensation Committee granted restricted stock units covering an aggregate of 64,635 shares of the Company’s common stock which generally vest annually over four years. Subsequent to December 31, 2017 and as of February 9, 2018, the Company repurchased 0.3 million shares of its common stock at an average cost of $37.03 per share for a total expenditure of $9.9 million .

Selected Quarterly Data (Unaudi

Selected Quarterly Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Data (Unaudited)	SELECTED QUARTERLY DATA (UNAUDITED) The following unaudited quarterly consolidated statements of operations for each of the quarters in the years ended December 31, 2017 and 2016 have been prepared on a basis consistent with the Company’s audited annual financial statements and include, in the opinion of management, all normal recurring adjustments necessary for the fair statement of the financial information contained in these statements. Quarter Ended (in thousands, except per share data) Mar. 31, June 30, Sept. 30, Dec. 31, Mar. 31, June 30, Sept. 30, Dec. 31, Revenue $ 99,324 $ 107,013 $ 107,758 $ 109,029 $ 111,582 $ 116,651 $ 121,796 $ 131,956 Cost of revenue 31,650 35,955 33,369 34,778 33,949 35,321 35,708 37,889 Gross profit 67,674 71,058 74,389 74,251 77,633 81,330 86,088 94,067 Operating expenses: Sales and marketing 56,701 57,835 53,690 57,405 56,894 62,073 60,554 60,750 Research and development 11,015 11,782 12,130 12,050 13,411 14,684 16,389 17,491 General and administrative 16,465 16,538 18,608 19,345 20,476 23,141 21,249 19,723 Restructuring — — — — — — — 1,539 Amortization of certain acquired intangible assets 150 — — — — — — — Total operating expenses 84,331 86,155 84,428 88,800 90,781 99,898 98,192 99,503 Loss from operations (16,657 ) (15,097 ) (10,039 ) (14,549 ) (13,148 ) (18,568 ) (12,104 ) (5,436 ) Other income (expense): Interest income (expense) and other income (expense), net (1,051 ) (2,350 ) (2,131 ) (3,756 ) (2,492 ) (2,333 ) (2,248 ) (3,260 ) Loss before income tax provision (17,708 ) (17,447 ) (12,170 ) (18,305 ) (15,640 ) (20,901 ) (14,352 ) (8,696 ) Income tax provision (535 ) (141 ) (218 ) (313 ) (571 ) (364 ) (503 ) (308 ) Net loss $ (18,243 ) $ (17,588 ) $ (12,388 ) $ (18,618 ) $ (16,211 ) $ (21,265 ) $ (14,855 ) $ (9,004 ) Net loss per share, basic and diluted $ (0.33 ) $ (0.32 ) $ (0.22 ) $ (0.33 ) $ (0.29 ) $ (0.37 ) $ (0.26 ) $ (0.16 ) Weighted average common shares outstanding, basic and diluted 54,827 55,278 55,964 56,300 56,642 56,935 57,627 57,826

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are presented in accordance with accounting standards generally accepted in the United States of America (“GAAP”), and include the accounts of Cornerstone OnDemand, Inc. and its wholly owned subsidiaries. All significant inter-company transactions and balances have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates. On an on-going basis, management evaluates its estimates, including among others those related to: (i) the realization of tax assets and estimates of tax liabilities and reserves, (ii) the recognition and disclosure of contingent liabilities, (iii) the collectability of accounts receivable, (iv) the evaluation of revenue recognition criteria, including the determination of standalone value and estimates of the selling price of multiple-deliverables in the Company’s revenue arrangements, (v) fair values of investments in marketable securities and strategic investments carried at fair value, (vi) the fair values of acquired assets and assumed liabilities in business combinations, (vii) the useful lives of property and equipment, capitalized software and intangible assets, (viii) impairment of long-lived assets, including goodwill, (ix) the amount and period of amortization of the commission payments to record to expense in proportion to the revenue that is recognized, (x) assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options and (xi) assumptions used in the valuation of various types of performance-based awards. These estimates are based on historical data and experience, as well as various other factors that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. The Company engages third-party valuation specialists to assist with the allocation of the purchase price in business combinations. Such estimates required the selection of appropriate valuation methodologies and models and significant judgment in evaluating ranges of assumptions and financial inputs.
Business Combinations	Business Combinations The results of businesses acquired in a business combination are included in the Company’s consolidated financial statements from the date of the acquisition. Purchase accounting results in assets and liabilities of an acquired business being recorded at their estimated fair values on the acquisition date. Any excess consideration over the fair value of assets acquired and liabilities assumed is recognized as goodwill. The Company performs valuations of assets acquired and liabilities assumed for an acquisition and allocates the purchase price to its respective net tangible and intangible assets. Determining the fair value of assets acquired and liabilities assumed requires management to use significant judgment and estimates including the selection of valuation methodologies, estimates of future revenue and cash flows, discount rates and selection of comparable companies. The Company engages the assistance of valuation specialists in concluding on fair value measurements in connection with determining fair values of assets acquired and liabilities assumed in a business combination. Transaction costs associated with business combinations are expensed as incurred, and are included in general and administrative expenses in the consolidated statement of operations.
Revenue Recognition	Revenue Recognition The Company derives its revenue from the following sources: • Subscriptions to the Company’s products and other offerings on a recurring basis —Clients pay subscription fees for access to the Company’s enterprise human capital management platform, other products and support on a recurring basis. Fees are based on a number of factors, including the number of products purchased, which may include e-learning content, and the number of users having access to a product. The Company generally recognizes revenue from subscriptions ratably over the term of the agreements. • Professional services and other —The Company offers its clients assistance in implementing its products and optimizing their use. Professional services include application configuration, system integration, business process re-engineering, change management and training services. Services are generally billed on a fixed fee basis and to a lessor degree on a time-and-material basis. These services are generally purchased as part of a subscription arrangement and are typically performed within the first several months of the arrangement. Clients may also purchase professional services at any other time. The Company generally recognizes revenue from fixed fee professional services using the proportional performance method over the period the services are performed and as time is incurred for time-and-material arrangements. The Company recognizes revenue when: (i) persuasive evidence of an arrangement for the sale of the Company’s products or professional services exists, (ii) the products have been made available or delivered, or services have been performed, (iii) the sales price is fixed or determinable and (iv) collectability is reasonably assured. The timing and amount the Company recognizes as revenue is determined based on the facts and circumstances of each client arrangement. Evidence of an arrangement consists of a signed client agreement. The Company considers that delivery of a product has commenced once it provides the client with log-in information to access and use the product. If non-standard acceptance periods or non-standard performance criteria exist, revenue recognition commences upon the satisfaction of the non-standard acceptance or performance criteria, as applicable. Standard acceptance or performance clauses relate to the Company’s products meeting certain perfunctory operating thresholds. Fees are fixed based on stated rates specified in the client agreement. If collectability is not considered reasonably assured, revenue is deferred until the fees are collected. The majority of client arrangements include multiple deliverables, such as subscriptions to the Company’s software products and professional services. The Company therefore recognizes revenue in accordance with the guidance for arrangements with multiple deliverables under Accounting Standards Update (“ASU”) 2009-13 “ Revenue Recognition (Topic 605)—Multiple-Deliverable Revenue Arrangements—a Consensus of the Emerging Issues Task Force ,” or ASU 2009-13. As clients do not have the right to the underlying software code for the products, the Company’s revenue arrangements are outside the scope of software revenue recognition guidance. The Company’s agreements generally do not contain any cancellation or refund provisions other than in the event of the Company’s default. For multiple-deliverable revenue arrangements, the Company first assesses whether each deliverable has value to the client on a standalone basis. The Company has determined that the products have standalone value, because, once access is given to a client, the products are fully functional and do not require any additional development, modification or customization. Professional services have standalone value because third-party service providers, distributors or clients themselves can perform these services without the Company’s involvement. The professional services assist clients with the configuration and integration of the Company’s products. The performance of these services generally does not require highly specialized or skilled individuals and are not essential to the functionality of the products. Based on the standalone value of the deliverables, and since clients do not have a general right of return relative to the included professional services, the Company allocates revenue among the separate deliverables in an arrangement under the relative selling price method using the selling price hierarchy established in ASU 2009-13. This hierarchy requires the selling price of each deliverable in a multiple deliverable arrangement to be based on, in descending order: (i) vendor-specific objective evidence of fair value (“VSOE”), (ii) third-party evidence of fair value (“TPE”) or (iii) management’s best estimate of the selling price (“BESP”). The Company is generally not able to determine VSOE or TPE for its deliverables, because the deliverables are sold separately and within a sufficiently narrow price range only infrequently, and because management has determined that there are no third-party offerings reasonably comparable to the Company’s products. Accordingly, total contract values are allocated to subscriptions to the products and professional services based on BESP. However, the amounts allocated to professional services generally do not exceed the contractually stated values of the professional services, as the revenue for subscriptions to the Company’s products is delivered over a longer period of time and is contingent upon delivery. This can result in higher allocations of the total contract value to subscriptions to the Company’s products over and above the relative selling price allocation based on this contingent revenue limitation. The determination of BESP requires the Company to make significant estimates and judgments. The Company considers numerous factors, including the nature of the deliverables themselves; the geography, market conditions and competitive landscape for the sale; internal costs; and pricing and discounting practices. The Company updates its estimates of BESP on an ongoing basis through internal periodic reviews and as events or circumstances may require. After the contract value is allocated to each deliverable in a multiple deliverable arrangement based on the relative selling price method, revenue is recognized for each deliverable based on the pattern in which the revenue is earned. For subscriptions to the products, revenue is recognized on a straight-line basis over the subscription term, which is typically three years. For professional services, revenue is recognized using the proportional performance method over the period the services are performed. For e-learning content and hosting, revenue is recognized ratably over the period the content is delivered or hosting service is provided. In a limited number of cases, the client’s intended use of a product requires enhancements to its underlying features and functionality. In some of these cases, revenue is recognized as one unit of accounting on a straight-line basis from the point at which the enhancements have been made to the product, through the remaining term of the agreement. In other cases where the enhancement is not required for the client’s intended use, revenue is recognized separately for the enhancement and the product. The enhancement revenue is recognized based on the allocated value on a straight-line basis once the enhancement has been made to the product, through the remaining term of the agreement. For arrangements in which the Company resells third-party e-learning training content to clients or hosts client or third-party e-learning training content provided by the client, revenue is recognized in accordance with accounting guidance as to when to report gross revenue as a principal or report net revenue as an agent. The Company recognizes third-party content revenue at the gross amount invoiced to clients when (i) the Company is the primary obligor, (ii) the Company has latitude to establish the price charged and (iii) the Company bears the credit risk in the transaction. For arrangements involving the sale of third-party content, clients are charged for the content based on pay-per-use or a fixed rate for a specified number of users, and revenue is recognized at the gross amount invoiced as the content is delivered. For arrangements where clients purchase third-party content directly from a third-party vendor, or provide it themselves, and the Company integrates the content into a product, the Company charges a fee per user or fee based on estimated bandwidth. In such cases, the fees are recognized at the net amount charged by the Company for hosting services as the content is delivered. The Company records amounts that have been invoiced to its clients in accounts receivable and in either deferred revenue or revenue depending on whether the revenue recognition criteria described above have been met. Deferred revenue that will be recognized during the succeeding twelve month period from the respective balance sheet date is recorded as current deferred revenue and the remaining portion is recorded as noncurrent.
Cost of Revenue	Cost of Revenue Cost of revenue consists primarily of costs related to hosting the Company’s products; personnel and related expenses, including stock-based compensation, and related expenses for network infrastructure, IT support, delivery of contracted professional services and on-going client support staff; payments to external service providers contracted to perform implementation services; depreciation of data centers; amortization of capitalized software costs; amortization of developed technology software license rights; content and licensing fees; and referral fees. In addition, the Company allocates a portion of overhead, such as rent, IT costs, depreciation and amortization and employee benefits costs, to cost of revenue based on headcount. Costs associated with providing professional services are recognized as incurred when the services are performed. Out-of-pocket travel costs related to the delivery of professional services are typically reimbursed by the client and are accounted for as both revenue and expense in the period in which the cost is incurred.
Commission Payments	Commission Payments The Company defers commissions paid to its sales force and related payroll taxes because these amounts are recoverable from the future revenue due to the non-cancelable client agreements that gave rise to the commissions. Commissions are deferred on the balance sheet and are recognized as sales and marketing expense over the term of the client agreement in proportion to the revenue that is recognized. Commissions are considered direct and incremental costs to client agreements and the Company generally commences payment of commissions within 45 to 75 days after execution of client agreements.
Research and Development	Research and Development Research and development expenses consist primarily of personnel and related expenses for the Company’s research and development staff, including salaries, benefits, bonuses and stock-based compensation; the cost of certain third-party service providers; and allocated overhead. Research and development expenses, other than software development costs qualifying for capitalization, are expensed as incurred.
Advertising	Advertising Advertising expenses for 2017 , 2016 and 2015 were $9.0 million , $6.6 million and $5.4 million , respectively, and are expensed as incurred.
Stock-based Compensation	Stock-Based Compensation The Company accounts for stock-based awards granted to employees and directors by recording compensation expense based on the awards’ estimated fair values. The Company grants stock options and restricted stock units that vest over time based on the continuing employment of the employee, as well as restricted stock units that vest based on meeting certain performance targets. The Company estimates the fair value of its restricted stock units based on the closing price of its common stock as of the date of grant. The Company estimates the fair value of its stock options as of the date of grant using the Black-Scholes option-pricing model. Determining the fair value of stock options under this model requires judgment, including estimating (i) the value per share of our common stock, (ii) volatility, (iii) the term of the awards, (iv) the dividend yield and (v) the risk-free interest rate. The assumptions used in calculating the fair value of stock based awards represent the Company’s best estimates, based on management’s judgment and subjective future expectations. These estimates involve inherent uncertainties. If any of the assumptions used in the model change significantly, stock-based compensation recorded for future awards may differ materially from that recorded for awards granted previously. Beginning in 2017, the Company began estimating expected volatility based solely on its historical volatility as a public company. In previous years, the Company estimated this using the average volatility of similar publicly traded companies as sufficient trading history of the Company’s stock was not available. For purposes of determining the expected term of the awards in the absence of sufficient historical data relating to stock option exercises for the Company, it applies a simplified approach in which the expected term of an award is presumed to be the mid-point between the vesting date and the expiration date of the award. The risk-free interest rate for periods within the expected life of an award, as applicable, is based on the United States Treasury yield curve in effect during the period the award was granted. The estimated dividend yield is zero, as the Company has not declared dividends in the past and does not currently intend to declare dividends in the foreseeable future. Once the Company has determined the estimated fair value of its stock-based awards, it recognizes the portion of that value that corresponds to the portion of the award that is ultimately expected to vest, taking estimated forfeitures into account. This amount is recognized as an expense over the vesting period of the award using the straight-line method for awards which contain only service conditions, and using the graded vesting method based upon the probability of the performance condition being met for awards which contain performance conditions. The Company estimates forfeitures based upon its historical experience and for each period, the Company reviews the estimated forfeiture rate and makes changes as factors affecting the forfeiture rate calculations and assumptions change. In addition, the Company has issued performance-based restricted stock units that vest based upon continued service over the vesting term and achievement of certain market conditions and performance goals, and others that vest based upon continued service over the vesting term and achievement of certain market conditions or performance goals, established by the Board of Directors, for a predetermined period. The fair value of the performance-based awards containing a market condition are determined using a Monte-Carlo simulation model that factors in the probability of the award vesting. The Company recognizes the fair value of stock-based compensation for awards which contain market-based conditions using the graded vesting method regardless of whether the market based condition is met. The fair value of the performance-based awards containing only a service and performance condition are determined based upon the closing price of the Company’s common stock on the date of the grant and the Company recognizes the fair value of awards containing a performance condition only if it is probable the performance condition will be met. For all performance-based awards, the fair value is not determined until all of the terms and conditions of the award are established.
Capitalized Software Costs	Capitalized Software Costs The Company capitalizes the costs associated with software developed or obtained for internal use, including costs incurred in connection with the development of its products, when the preliminary project stage is completed, management has decided to make the project a part of its future offering and the software will be used to perform the function intended. These capitalized costs include external direct costs of materials and services consumed in developing or obtaining internal-use software, personnel and related expenses for employees who are directly associated with and who devote time to internal-use software projects and, when material, interest costs incurred during the development. Capitalization of these costs ceases once the project is substantially complete and the software is ready for its intended purpose. Costs incurred for upgrades and enhancements to the products are also capitalized. Post-configuration training and maintenance costs are expensed as incurred. Capitalized software costs are amortized to cost of revenue using the straight-line method over an estimated useful life of the software, which is typically three years , commencing when the software is ready for its intended use. The Company does not transfer ownership of or lease its software to its clients.
Comprehensive Loss	Comprehensive Loss Comprehensive loss encompasses all changes in equity other than those arising from transactions with stockholders, and consists of net loss, currency translation adjustments and unrealized gains or losses on investments. For the years ended December 31, 2017 , 2016 and 2015 , accumulated other comprehensive income (loss) comprised a cumulative translation adjustment and also included net unrealized gains (losses) on investments.
Income Taxes	Income Taxes The Company uses the liability method of accounting for income taxes. Under the liability method, deferred taxes are determined based on the temporary differences between the financial statement and tax bases of assets and liabilities, using tax rates expected to be in effect during the years in which the bases differences are expected to reverse. A valuation allowance is recorded when it is more likely than not that some of the deferred tax assets will not be realized. In determining the need for valuation allowances, the Company considers projected future taxable income and the availability of tax planning strategies. The Company has recorded a full valuation allowance to reduce its United States, United Kingdom, New Zealand, Hong Kong and Brazil net deferred tax assets to zero, as it has determined that it is not more likely than not that these deferred tax assets will be realized. The Company has assessed its income tax positions and recorded tax benefits for all years subject to examination, based upon its evaluation of the facts, circumstances and information available at each period end. For those tax positions where the Company has determined there is a greater than 50% likelihood that a tax benefit will be sustained, the Company has recorded the largest amount of tax benefit that may potentially be realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. For those income tax positions where it is determined there is less than 50% likelihood that a tax benefit will be sustained, no tax benefit has been recognized.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers cash and cash equivalents to include short-term, highly liquid investments that are readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value, including investments with original or remaining maturities from the date of purchase of three months or less.
Investments in Marketable Securities	Investments in Marketable Securities The Company’s available-for-sale investments in marketable securities are recorded at fair value, with any unrealized gains and losses, net of taxes, reported as a component of stockholders’ equity until realized or until a determination is made that an other-than-temporary decline in market value has occurred. If the Company determines that an other-than-temporary decline has occurred for debt securities that the Company does not then currently intend to sell, the Company recognizes the credit loss component of an other-than-temporary impairment in other income (expense) and the remaining portion in other comprehensive income (loss). The credit loss component is identified as the amount of the present value of cash flows not expected to be received over the remaining term of the security, based on cash flow projections. In determining whether an other-than-temporary impairment exists, the Company considers: (i) the length of time and the extent to which the fair value has been less than cost; (ii) the financial condition and near-term prospects of the issuer of the securities; and (iii) the Company’s intent and ability to retain the security for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of marketable securities sold is determined based on the specific identification method and any realized gains or losses on the sale of investments are reflected as a component of interest income or expense. In addition, the Company classifies marketable securities as current or non-current based upon the maturity dates of the securities.
Strategic Investments	Strategic Investments Since 2014, the Company has invested in equity securities of multiple privately-held companies. The Company accounted for each of these investment using the cost method of accounting, as we do not have significant influence or a controlling financial interest over these entities. These investments are subject to periodic impairment reviews and are considered to be impaired when a decline in fair value is judged to be other-than-temporary. These investments are included in long-term investments on the Consolidated Balance Sheets.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts The Company bases its allowance for doubtful accounts on its historical collection experience and a review in each period of the status of the then-outstanding accounts receivable.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are recorded at historical cost, less accumulated depreciation and amortization. Depreciation is computed using the straight-line method based upon the estimated useful lives of the assets, generally two to seven years (See Note 6 ). The Company leases equipment under capital lease arrangements. The assets and liabilities under capital lease are recorded at the lesser of the present value of aggregate future minimum lease payments, including estimated bargain purchase options, or the fair value of the asset under lease. Assets under capital lease are depreciated using the straight-line method over the lesser of the estimated useful life of the asset or the term of the lease. Leasehold improvements are depreciated on a straight-line basis over the shorter of their estimated useful lives or lease terms. Repair and maintenance costs are charged to expense as incurred, while renewals and improvements are capitalized.
Impairment of Long Lived Assets	Impairment of Long Lived Assets The Company evaluates the recoverability of its long-lived assets with finite useful lives, including intangible assets, for impairment whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. Such triggering events or changes in circumstances may include: a significant decrease in the market price of a long-lived asset, a significant adverse change in the extent or manner in which a long-lived asset is being used, a significant adverse change in legal factors or in the business climate, the impact of competition or other factors that could affect the value of a long-lived asset, a significant adverse deterioration in the amount of revenue or cash flows expected to be generated from an asset group, an accumulation of costs significantly in excess of the amount originally expected for the acquisition or development of a long-lived asset, current or future operating or cash flow losses that demonstrate continuing losses associated with the use of a long-lived asset, or a current expectation that, more likely than not, a long-lived asset will be sold or otherwise disposed of significantly before the end of its previously estimated useful life. The Company performs impairment testing at the asset group level that represents the lowest level for which identifiable cash flows are largely independent of the cash flows of other assets and liabilities. If events or changes in circumstances indicate that the carrying amount of an asset group may not be recoverable and the expected undiscounted future cash flows attributable to the asset group are less than the carrying amount of the asset group, an impairment loss equal to the excess of the asset’s carrying value over its fair value is recorded. Fair value is determined based upon estimated undiscounted future cash flows.
Intangible Assets	Intangible Assets Identifiable intangible assets primarily consist of trade names and intellectual property and acquisition-related intangibles, including developed technology, customer relationships, non-compete agreements, patents, trade names and trademarks. The Company determines the appropriate useful life of its intangible assets by performing an analysis of expected cash flows of the acquired assets. Intangible assets are amortized over their estimated useful lives ranging from two to ten years, generally using the straight line method which approximates the pattern in which the economic benefits are consumed.
Goodwill	Goodwill Goodwill is not amortized, but instead is required to be tested for impairment annually and under certain circumstances. The Company performs such testing of goodwill in the fourth quarter of each year, or as events occur or circumstances change that would more likely than not reduce the fair value of a reporting unit below its carrying amount. Events or changes in circumstances which could trigger an impairment review include a significant adverse change in legal factors or in the business climate, an adverse action or assessment by a regulator, unanticipated competition, a loss of key personnel, significant changes in the manner of the Company’s use of the acquired assets or the strategy for the Company’s overall business, significant negative industry or economic trends, or significant underperformance relative to expected historical or projected future results of operations. As part of the annual impairment test, the Company may conduct an assessment of qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If the Company elects not to perform the qualitative assessment or it determines that it is more likely than not that the fair value of a reporting unit is less than its carrying amount, it then conducts the first step of a two-step impairment test. The first step of the test for goodwill impairment compares the fair value of the applicable reporting unit with its carrying value. Fair value was determined using a market approach, which includes consideration of the Company’s own market capitalization. If the fair value of a reporting unit is less than the reporting unit’s carrying value, the Company performs the second step of the test for impairment of goodwill in which the Company compares the implied fair value of the reporting unit’s goodwill with the carrying value of that goodwill. The estimate of implied fair value of goodwill may require valuations of certain internally generated and unrecognized intangible assets and other assets and liabilities. If the carrying value of the goodwill exceeds the calculated implied fair value, the excess amount will be recognized as an impairment loss.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value represents the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The fair value hierarchy is based on the following three levels of inputs, of which the first two are considered observable and the last one is considered unobservable: • Level 1—Quoted prices (unadjusted) in active markets for identical assets or liabilities that management has the ability to access at the measurement date. • Level 2—Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. • Level 3—Unobservable inputs. Observable inputs are based on market data obtained from independent sources.
Concentration of Risk	Concentration of Risk Financial instruments that potentially subject the Company to concentration of credit risk consist principally of cash, cash equivalents, restricted cash and accounts receivable. The Company’s cash and cash equivalents are deposited with several financial institutions and, at times, may exceed federally insured limits, as applicable. The Company performs ongoing credit evaluations of its clients.
Foreign Currency Transactions and Translation	Foreign Currency Transactions and Translation Transactions in foreign currencies are translated into U.S. Dollars at the rates of exchange in effect at the date of the transaction. Unrealized transaction (losses) gains were approximately $(3.1) million , $20 thousand and $(0.9) million for the years ended December 31, 2017 , 2016 and 2015 , respectively, and are included in other, net within other income (expense), net, in the accompanying consolidated statements of operations. The Company has entities in various countries. For entities where the local currency is different than the functional currency, the local currency financial statements have been remeasured from the local currency into the functional currency using the current exchange rate for monetary accounts and historical exchange rates for nonmonetary accounts, with exchange differences on remeasurement included in other income (loss). To the extent that the functional currency of our subsidiaries is different than the U.S Dollar, the financial statements have then been translated into U.S. Dollars using period-end exchanges rates for assets and liabilities and average exchanges rates for the results of operations. Foreign currency translation gains and losses are included as a component of Accumulated other comprehensive income or loss in the Consolidated Balance Sheets.
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In January 2017, the Financial Accounting Standards Board (“FASB”) issued a new accounting standards update (“ASU”) which amends the reporting requirement in regard to new accounting pronouncements or existing pronouncements that have not yet been adopted. The guidance requires registrants to disclose the effect that recently issued accounting standards will have on their financial statements when adopted in a future period. The Company implemented this requirement as of the beginning of the first quarter of 2017. In March 2016, the FASB issued a new ASU to simplify several areas of accounting for share-based compensation arrangements, including the income tax impact, classification on the statement of cash flows and forfeitures. The Company adopted this ASU as of the beginning of the first quarter of 2017 and has elected to continue to estimate expected forfeitures over the course of a vesting period. Further, the ASU eliminates the requirement to delay the recognition of excess tax benefits until they reduce current taxes payable. Upon adoption on January 1, 2017, the Company recorded a $39.4 million U.S. deferred tax asset for the previously unrecognized excess tax benefits, however, this had no impact on our accumulated deficit balance as the deferred tax assets were fully offset by a valuation allowance. The adoption did not have any other material impacts on the Company’s financial statements. Recent Accounting Pronouncements In May 2017, the FASB issued a new ASU, which amends the scope of modification accounting for share-based payment arrangements. It provides guidance on the types of changes to the terms or conditions of share-based payment awards to which an entity would be required to apply modification accounting under ASC 718. This guidance is effective for the Company’s interim and annual reporting periods beginning January 1, 2018. The Company does not expect the adoption of this ASU to have a material impact on its financial statements. In August 2016, the FASB issued a new ASU to clarify how companies present and classify certain cash receipts and cash payments in the statement of cash flows. This guidance is effective for our interim and annual reporting periods beginning January 1, 2018. The Company does not expect the adoption of this ASU to have a material impact on its financial statements. In February 2016, the FASB issued a new ASU, which amends a number of aspects of lease accounting, including requiring lessees to recognize operating leases with a term greater than one year on their balance sheet as a right-of-use asset and corresponding lease liability, measured at the present value of the lease payments. This guidance is effective for the Company’s interim and annual reporting periods beginning January 1, 2019. Upon adoption, the Company expects additional lease liability to be recognized on the consolidated balance sheets. In January 2016, the FASB issued a new ASU that provides guidance for the recognition, measurement, presentation and disclosure of financial assets and liabilities. This guidance is effective for the Company’s interim and annual reporting periods beginning January 1, 2018. The Company does not expect the adoption of this ASU to have a material impact on its financial statements. In May 2014, the FASB issued a new ASU that provides guidance for recognizing revenue from contracts with customers. Under the new standard, the Company is required to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects consideration that the Company expects to be entitled to in exchange for those goods or services. The standard permits the use of the full retrospective method, in which case the standard would be applied to each prior reporting period presented and the cumulative effect of applying the standard would be recognized at the earliest period shown, or the modified retrospective method, in which case the cumulative effect of applying the standard would be recognized at the date of initial application. The Company has evaluated the transition methods and elected to use the modified retrospective method and will adopt this standard beginning January 1, 2018. During fiscal year 2017, the Company evaluated the accounting and disclosure requirements of the standard and implemented the appropriate changes to its business processes, systems and internal controls to enable the preparation of its financial information on adoption. The Company is in the process of finalizing its assessment of the standard as the Company’s analysis of costs that represent incremental costs to obtain a contract is ongoing. The Company believes it will finalize this analysis in connection with the preparation of its financial statements as of and for the quarter ended March 31, 2018. Costs that represent incremental costs to obtain a contract will be amortized over the expected product life. The Company has finalized its assessment of how it recognizes revenue for subscriptions to the Company’s products and other offerings on a recurring basis and revenue for professional services. Upon adoption, revenue for professional services will be based on the relative standalone selling price without limitation to its contractual value. Prior to adoption, such allocation was limited by its contractual value. This change is expected to result in a material increase in the aggregate amount allocated to professional services when allocating total contract values on a relative standalone selling price basis under the new standard. The Company expects to adjust the following balance sheet line items in fiscal year 2018 to reflect the adoption of the new ASU: deferred commissions; other assets, net; accrued expenses; deferred revenue, current portion; other non-current liabilities; deferred revenue, net of current portion; and accumulated deficit.

Summary of Significant Accoun27

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of black-scholes option-pricing model assumptions	The following information represents the weighted average of the assumptions used in the Black-Scholes option-pricing model for stock options granted during each of the last three years: For the Years Ended December 31, 2017 2016 2015 Risk-free interest rate n/a 1.4 % 1.8 % Expected term (in years) n/a 5.8 6.0 Estimated dividend yield n/a — % — % Estimated volatility n/a 48.8 % 41.8 %
Reconciliation of beginning and ending amount of allowance for doubtful accounts	A reconciliation of the beginning and ending amount of allowance for doubtful accounts for the years ended December 31, 2017 , 2016 and 2015 , is as follows (in thousands): 2017 2016 2015 Beginning balance, January 1 $ 3,532 $ 2,578 $ 2,177 Additions and adjustments 7,680 3,165 1,368 Write-offs (3,734 ) (2,211 ) (967 ) Ending balance, December 31 $ 7,478 $ 3,532 $ 2,578

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Basic and diluted loss per share	The following table presents the Company's basic and diluted net loss per share (in thousands, except per share amounts): For the Years Ended December 31, 2017 2016 2015 Net loss $ (61,335 ) $ (66,837 ) $ (85,516 ) Weighted-average shares of common stock outstanding 57,262 55,595 54,171 Net loss per share — basic and diluted $ (1.07 ) $ (1.20 ) $ (1.58 )
Anti-dilutive shares excluded from calculation of net loss per share	At December 31, 2017 , 2016 and 2015 , the following potential shares were excluded from the computation of diluted net loss per share because their effect would have been anti-dilutive (in thousands): December 31, 2017 2016 2015 Options to purchase common stock, restricted stock units and performance-based restricted stock units 10,143 10,635 10,860 Shares issuable pursuant to employee stock purchase plan 114 89 77 Convertible notes 11,825 4,682 4,682 Common stock warrants 4,682 4,682 4,682 Total shares excluded from net loss per share 26,764 20,088 20,301

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Schedule of marketable securities	The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2017 (in thousands): December 31, 2017 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash Equivalent Investments Money market funds $ 358,859 $ — $ — $ 358,859 $ 358,859 $ — Certificate of deposit 10,000 — — 10,000 10,000 — Corporate bonds 74,868 — (220 ) 74,648 — 74,648 U.S. treasury securities 189,310 — (430 ) 188,880 — 188,880 $ 633,037 $ — $ (650 ) $ 632,387 $ 368,859 $ 263,528 The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2016 (in thousands): December 31, 2016 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Cash Equivalent Investments Money market funds $ 48,136 $ — $ — $ 48,136 $ 48,136 $ — Corporate bonds 60,725 1 (50 ) 60,676 — 60,676 Agency bonds 28,954 2 (26 ) 28,930 — 28,930 U.S. treasury securities 157,829 17 (160 ) 157,686 — 157,686 Commercial paper 10,473 — — 10,473 — 10,473 $ 306,117 $ 20 $ (236 ) $ 305,901 $ 48,136 $ 257,765

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of finite-lived intangible assets	The following table presents the gross carrying amount and accumulated amortization of finite-lived intangible assets as of December 31, 2017 and 2016 (in thousands): December 31, 2017 December 31, 2016 Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Developed technology $ 29,984 $ (29,984 ) $ — $ 29,984 $ (22,711 ) $ 7,273 Software license rights 1,654 (1,654 ) — 1,654 (1,506 ) 148 Total $ 31,638 $ (31,638 ) $ — $ 31,638 $ (24,217 ) $ 7,421

Other Balance Sheet Amounts (Ta

Other Balance Sheet Amounts (Tables)	12 Months Ended
	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Schedule of property and equipment, net	The balance of property and equipment, net is as follows (in thousands): Useful Life December 31, 2017 2016 Computer equipment and software 3 – 5 years $ 38,838 $ 32,926 Furniture and fixtures 7 years 3,855 3,837 Leasehold improvements 2 – 6 years 10,046 9,878 Renovation in progress n/a 58 58 52,797 46,699 Less: accumulated depreciation and amortization (31,980 ) (22,737 ) Total property and equipment, net $ 20,817 $ 23,962
Schedule of accrued expenses	The balance of accrued expenses is as follows (in thousands): December 31, 2017 2016 Accrued bonuses $ 13,557 $ 13,371 Accrued commissions 12,401 10,616 Other accrued expenses 31,570 23,632 Total accrued expenses $ 57,528 $ 47,619

Fair Value of Financial Instr32

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Assets and liabilities measured at fair value	Assets and liabilities measured at fair value on a recurring basis included the following as of December 31, 2017 and 2016 (in thousands): December 31, 2017 December 31, 2016 Fair Value Level 1 Level 2 Level 3 Fair Value Level 1 Level 2 Level 3 Cash equivalents $ 368,859 $ 358,859 $ 10,000 $ — $ 48,136 $ 48,136 $ — $ — Corporate bonds 74,648 — 74,648 — 60,676 — 60,676 — Agency bonds — — — — 28,930 — 28,930 — U.S. treasury securities 188,880 — 188,880 — 157,686 — 157,686 — Commercial paper — — — — 10,473 — 10,473 — $ 632,387 $ 358,859 $ 273,528 $ — $ 305,901 $ 48,136 $ 257,765 $ —

Debt and Other Financing Arra33

Debt and Other Financing Arrangements (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of the net carrying amount of debt	The net carrying amounts of the liability components of the 2018 and 2021 Notes as of December 31, 2017 and 2016 consists of the following (in thousands): December 31, 2017 December 31, 2016 Principal amount $ 553,000 $ 253,000 Unamortized debt discount (10,190 ) (12,550 ) Net carrying amount before unamortized debt issuance costs 542,810 240,450 Unamortized debt issuance costs (9,617 ) (2,015 ) Net carrying value $ 533,193 $ 238,435
Schedule of note interest expense	The following table presents the interest expense recognized related to the 2018 Notes and the 2021 Notes for years ended December 31, 2017 , 2016 and 2015 (in thousands): Years Ended December 31, 2017 2016 2015 Contractual interest expense at 1.50% and 5.75% per annum $ 4,897 $ 3,795 $ 3,795 Amortization of debt issuance costs 1,472 1,263 1,202 Accretion of debt discount 8,360 7,867 7,489 Total $ 14,729 $ 12,925 $ 12,486

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of stock option activity	The Company has granted stock options, which vest upon meeting service conditions. The following table summarizes the stock option activity which contain only service conditions, under the Company’s 1999, 2009 and 2010 Plans (in thousands, except per share and term information): Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (1) Outstanding, December 31, 2016 6,041 $ 32.01 6.2 $ 74,989 Granted — — Exercised (413 ) 16.38 Forfeited (334 ) 35.71 Outstanding, December 31, 2017 5,294 32.99 5.3 40,122 Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term Aggregate Intrinsic Value (1) Exercisable at December 31, 2017 4,934 $ 32.60 5.2 $ 39,951 Vested and expected to vest at December 31, 2017 5,280 32.98 5.3 40,113 (1) Based on the Company’s closing stock price of $35.33 on December 31, 2017 and $42.31 on December 31, 2016.
Schedule of stock options outstanding and exercisable	The following table summarizes information about stock options, which contain only service conditions, under the Company’s equity incentive plans at December 31, 2017 (in thousands except term information): Options Outstanding at December 31, 2017 Options Exercisable at December 31, 2017 Number of Options Weighted Average Remaining Contractual Term (in years) Number of Options Weighted Average Remaining Contractual Term (in years) Range of Exercise Prices $0.34 to $1.65 92 1.9 92 1.9 $5.93 to $8.88 677 2.9 677 2.9 $12.54 to $15.41 132 3.7 132 3.7 $16.24 to $18.82 305 4.0 305 4.0 $20.85 to $23.94 552 4.4 552 4.4 $27.55 to $31.44 207 5.6 193 5.5 $31.64 to $36.15 835 6.8 633 6.6 $38.03 to $45.76 1,093 6.1 1,036 6.0 $46.20 to $56.05 1,401 6.1 1,314 6.1 5,294 5.3 4,934 5.2
Schedule of restricted stock unit activity	Restricted stock unit activity for the year ended December 31, 2017 under the Company’s equity incentive plans is summarized as follows (shares in thousands): Number of Shares Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 3,254 $ 36.35 Granted 1,938 37.99 Forfeited (367 ) 36.04 Vested (1,035 ) 36.34 Outstanding at December 31, 2017 3,790 $ 37.22
Summary of issuances of awards under new compensation award structure	The following table summarizes the Company’s issuances of awards under the new compensation award structure: Grant Date Performance Measures Vesting Term Performance Period # of Shares at Target # of Shares at Maximum Grant Date Fair Value per share July 2016 (a) the Company meeting certain revenue and cash flow targets through December 31, 2018 and (b) the recipient continuing to provide services to the Company through the end of June 2019 Three years Fiscal years 2016, 2017 and 2018 166,600 499,800 $ 38.67 March 2017 (a) the Company meeting certain revenue and cash flow targets through December 31, 2019 and (b) the recipient continuing to provide services to the Company through the end of March 2020 Three years Fiscal years 2017, 2018 and 2019 185,270 555,810 $ 41.73
Summary of stock-based compensation expense	Stock-based compensation expense related to stock options, restricted stock units, the ESPP and performance-based restricted stock units is included in the following line items in the accompanying Consolidated Statements of Operations for the years ended December 31, 2017 , 2016 and 2015 (in thousands): Years Ended December 31, 2017 2016 2015 Cost of revenue $ 4,904 $ 4,732 $ 3,887 Sales and marketing 28,427 25,642 23,604 Research and development 9,630 7,586 6,010 General and administrative 22,869 16,739 9,580 Total $ 65,830 $ 54,699 $ 43,081

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Summary of income before income tax, domestic and foreign	The components of the Company’s loss before provision (benefit) for income taxes are as follows (in thousands): Years Ended December 31, 2017 2016 2015 United States $ (32,853 ) $ (39,107 ) $ (59,797 ) Foreign (26,736 ) (26,523 ) (24,538 ) Loss before provision for income taxes $ (59,589 ) $ (65,630 ) $ (84,335 )
Components of income tax provision (benefit)	The components of the provision (benefit) for income taxes attributable to continuing operations are as follows (in thousands): Years Ended December 31, 2017 2016 2015 Current income tax provision: Federal $ — $ — $ — State 114 105 147 Foreign 1,580 1,838 1,139 Total current income tax provision 1,694 1,943 1,286 Deferred income tax benefit: Federal — — — State — — — Foreign 52 (736 ) (105 ) Total deferred income tax benefit 52 (736 ) (105 ) Total income tax provision (benefit) $ 1,746 $ 1,207 $ 1,181
Schedule of effective income tax rate reconciliation	The differences in the total provision for income taxes that would result from applying the 34% federal statutory rate to loss before provision for income taxes and the reported provision for income taxes are as follows (in thousands): Years Ended December 31, 2017 2016 2015 U.S. Federal tax benefit at statutory rates $ (20,260 ) $ (22,310 ) $ (28,681 ) State income taxes, net of federal tax benefit (806 ) (855 ) (1,632 ) Foreign rate differential 5,220 3,711 3,964 Stock based compensation 3,182 4,467 4,673 Other permanent differences (494 ) (750 ) (99 ) Deferred adjustments / U.S. rate change 7,811 — — Other 262 1,494 536 Valuation allowance 6,831 15,450 22,420 Total income tax (benefit) provision $ 1,746 $ 1,207 $ 1,181
Schedule of the components of deferred tax assets and liabilities	Major components of the Company’s deferred tax assets (liabilities) at December 31, 2017 and 2016 are as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Accrued expenses $ 2,371 $ 3,037 Long-lived intangible assets and fixed assets — basis difference 19,884 22,146 Net operating loss carryforwards 80,615 68,356 Stock-based compensation 16,886 19,515 Deferred revenue 2,739 4,060 Convertible note hedge 1,467 6,387 Other 2,721 2,276 Total deferred tax assets 126,683 125,777 Valuation allowance (118,606 ) (111,775 ) Deferred tax assets, net of valuation allowance 8,077 14,002 Deferred tax liabilities: Prepaid expenses and deferred commissions (5,672 ) (7,718 ) Convertible note discount (1,074 ) (4,721 ) Other (410 ) (591 ) Total deferred tax liabilities (7,156 ) (13,030 ) Net deferred tax assets (liabilities) $ 921 $ 972
Summary of the reconciliation of unrecognized tax benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits for the years ended December 31, 2017 , 2016 and 2015 is as follows (in thousands): Years Ended December 31, 2017 2016 2015 Balance at January 1 $ 276 $ 276 $ 276 Additions for tax positions related to the current year 995 — — Balance at December 31 $ 1,271 $ 276 $ 276

Segment and Geographic Inform36

Segment and Geographic Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule of revenue by source	The following table sets forth the Company’s sources of revenue (dollars in thousands): At or For Year Ended December 31, 2017 2016 2015 Subscription revenue $ 396,764 $ 339,756 $ 270,093 Percentage of subscription revenue to total revenue 82.3 % 80.3 % 79.5 % Professional services revenue $ 85,221 $ 83,368 $ 69,558 Percentage of professional services revenue to total revenue 17.7 % 19.7 % 20.5 % $ 481,985 $ 423,124 $ 339,651
Schedule of revenue from external customers and long-lived assets, by geographical areas	Revenue by geographic region, which is generally based on the address of the Company’s clients as defined in their master subscription agreements, is set forth below (in thousands): Years Ended December 31, 2017 2016 2015 Revenue United States $ 313,729 $ 284,657 $ 228,724 United Kingdom 25,701 27,571 30,104 All other countries 142,555 110,896 80,823 Total revenue $ 481,985 $ 423,124 $ 339,651 Property and equipment by region is set forth below (in thousands): December 31, 2017 2016 Property and equipment, net United States $ 16,468 $ 19,843 United Kingdom 3,378 2,985 All other countries 971 1,134 Total property and equipment, net $ 20,817 $ 23,962

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum lease payments	Future minimum lease payments under non-cancelable operating leases at December 31, 2017 are as follows (in thousands): Operating Leases 2018 $ 8,246 2019 826 2020 84 2021 73 2022 — Total minimum lease payments $ 9,229

Selected Quarterly Data (Unau38

Selected Quarterly Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly consolidated statements of operations	The following unaudited quarterly consolidated statements of operations for each of the quarters in the years ended December 31, 2017 and 2016 have been prepared on a basis consistent with the Company’s audited annual financial statements and include, in the opinion of management, all normal recurring adjustments necessary for the fair statement of the financial information contained in these statements. Quarter Ended (in thousands, except per share data) Mar. 31, June 30, Sept. 30, Dec. 31, Mar. 31, June 30, Sept. 30, Dec. 31, Revenue $ 99,324 $ 107,013 $ 107,758 $ 109,029 $ 111,582 $ 116,651 $ 121,796 $ 131,956 Cost of revenue 31,650 35,955 33,369 34,778 33,949 35,321 35,708 37,889 Gross profit 67,674 71,058 74,389 74,251 77,633 81,330 86,088 94,067 Operating expenses: Sales and marketing 56,701 57,835 53,690 57,405 56,894 62,073 60,554 60,750 Research and development 11,015 11,782 12,130 12,050 13,411 14,684 16,389 17,491 General and administrative 16,465 16,538 18,608 19,345 20,476 23,141 21,249 19,723 Restructuring — — — — — — — 1,539 Amortization of certain acquired intangible assets 150 — — — — — — — Total operating expenses 84,331 86,155 84,428 88,800 90,781 99,898 98,192 99,503 Loss from operations (16,657 ) (15,097 ) (10,039 ) (14,549 ) (13,148 ) (18,568 ) (12,104 ) (5,436 ) Other income (expense): Interest income (expense) and other income (expense), net (1,051 ) (2,350 ) (2,131 ) (3,756 ) (2,492 ) (2,333 ) (2,248 ) (3,260 ) Loss before income tax provision (17,708 ) (17,447 ) (12,170 ) (18,305 ) (15,640 ) (20,901 ) (14,352 ) (8,696 ) Income tax provision (535 ) (141 ) (218 ) (313 ) (571 ) (364 ) (503 ) (308 ) Net loss $ (18,243 ) $ (17,588 ) $ (12,388 ) $ (18,618 ) $ (16,211 ) $ (21,265 ) $ (14,855 ) $ (9,004 ) Net loss per share, basic and diluted $ (0.33 ) $ (0.32 ) $ (0.22 ) $ (0.33 ) $ (0.29 ) $ (0.37 ) $ (0.26 ) $ (0.16 ) Weighted average common shares outstanding, basic and diluted 54,827 55,278 55,964 56,300 56,642 56,935 57,627 57,826

Organization (Details)

Organization (Details)	12 Months Ended
	Dec. 31, 2017segment
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of operating segments	1

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Business Combinations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
General and administrative
Business Acquisition [Line Items]
Acquisition related costs	$ 0	$ 0	$ 0

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Commission Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting for Commission Payments
Commissions deferred during the period	$ 48,200	$ 33,300	$ 42,000
Deferred commissions	42,806	36,298
Sales and marketing
Accounting for Commission Payments
Amortization of deferred commissions	$ 41,700	$ 33,000	$ 32,300
Minimum
Accounting for Commission Payments
Commission payment period	45 days
Maximum
Accounting for Commission Payments
Commission payment period	75 days

Summary of Significant Accoun42

Summary of Significant Accounting Policies - Research and Development & Advertising (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Research and development	$ 17,491	$ 16,389	$ 14,684	$ 13,411	$ 12,050	$ 12,130	$ 11,782	$ 11,015	$ 61,975	$ 46,977	$ 40,991
Advertising expense									$ 9,000	$ 6,600	$ 5,400

Summary of Significant Accoun43

Summary of Significant Accounting Policies - Share-Based Compensation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Based Compensation - Black-Scholes Option-Pricing Model Assumptions:
Tax benefit from share based compensation	$ 0	$ 0
Stock options
Stock Based Compensation - Black-Scholes Option-Pricing Model Assumptions:
Risk-free interest rate		1.40%	1.80%
Expected term (in years)		5 years 9 months 15 days	5 years 11 months 18 days
Estimated dividend yield		0.00%	0.00%
Estimated volatility		48.80%	41.80%

Summary of Significant Accoun44

Summary of Significant Accounting Policies - Capitalized Software Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Software Costs
Capitalized software costs, estimated useful life	3 years
Capitalized software costs, amount capitalized during the period	$ 24.3	$ 20.9	$ 16.5
Amortization of capitalized software costs	17.6	$ 13.2	$ 9.1
Capitalized software costs, estimated amortization expense within the next year	19.1
Capitalized software costs, estimated amortization expense within two years	12.5
Capitalized software costs, estimated amortization expense within three years	5.7
Capitalized software costs, estimated amortization expense within four years	$ 0.1

Summary of Significant Accoun45

Summary of Significant Accounting Policies - Cash, Investments, and Restricted Cash (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents
Cash balances	$ 24,700,000	$ 35,200,000
Money market funds backed by U.S. Treasury Securities	358,900,000	48,100,000
Certificates of deposit	10,000,000	0
Investments in Marketable Securities
Marketable Securities	$ 263,500,000	$ 257,800,000

Summary of Significant Accoun46

Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts - Reconciliation:
Accounts receivable allowance, Beginning balance	$ 3,532	$ 2,578	$ 2,177
Additions and adjustments	7,680	3,165	1,368
Write-offs	(3,734)	(2,211)	(967)
Accounts receivable allowance, Ending balance	7,478	3,532	2,578
Bad debt expense recognized	$ 1,400	$ 800	$ 700

Summary of Significant Accoun47

Summary of Significant Accounting Policies - Property and Equipment, Net (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	7 years

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Impairment, Intangible Assets, and Goodwill (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Impairment charges related to identified intangible assets	$ 1,300,000	$ 0
Intangible Assets
Goodwill, impairment loss	$ 0	$ 0
Minimum
Intangible Assets
Identified intangible assets amortization period	2 years
Maximum
Intangible Assets
Identified intangible assets amortization period	10 years

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Convertible Notes (Details) - Convertible notes - USD ($)	Dec. 31, 2017	Jun. 17, 2013
The 2018 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued		1.50%
Debt issued, senior convertible notes		$ 253,000,000
The 2021 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued	5.75%
Debt issued, senior convertible notes	$ 300,000,000

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Foreign Currency Transactions and Translation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other income (expense), net
Foreign Currency Fair Value Hedge Derivative [Line Items]
Foreign currency transactions and translation	$ (3,100)	$ 20	$ (900)

Summary of Significant Accoun51

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Jan. 01, 2017	Dec. 31, 2016	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Previously unrecognized excess tax benefit	$ 39,400	$ 39,400
Valuation allowance		111,775	$ 118,606
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Valuation allowance		$ 39,400

Net Loss Per Share - Basic and

Net Loss Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss									$ (61,335)	$ (66,837)	$ (85,516)
Weighted-average shares of common stock outstanding (in shares)	57,826	57,627	56,935	56,642	56,300	55,964	55,278	54,827	57,262	55,595	54,171
Net loss per share — basic and diluted (usd per share)	$ (0.16)	$ (0.26)	$ (0.37)	$ (0.29)	$ (0.33)	$ (0.22)	$ (0.32)	$ (0.33)	$ (1.07)	$ (1.20)	$ (1.58)

Net Loss Per Share - Anti-dilut

Net Loss Per Share - Anti-dilutive Shares Excluded From Calculation of Net Loss Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded from net loss per share (in shares)	26,764	20,088	20,301
Options to purchase common stock, restricted stock units and performance-based restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded from net loss per share (in shares)	10,143	10,635	10,860
Shares issuable pursuant to employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded from net loss per share (in shares)	114	89	77
Convertible notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded from net loss per share (in shares)	11,825	4,682	4,682
Common stock warrants | Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares excluded from net loss per share (in shares)	4,682	4,682	4,682

Investments - Investments in Ma

Investments - Investments in Marketable Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 633,037	$ 306,117
Unrealized Gains	0	20
Unrealized Losses	(650)	(236)
Fair Value	$ 632,387	305,901
Investments, weighted average maturity	9 months
Investments in unrealized loss for more than 12 months | security	0
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 358,859	48,136
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	358,859	48,136
Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	10,000
Unrealized Gains	0
Unrealized Losses	0
Fair Value	10,000
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	74,868	60,725
Unrealized Gains	0	1
Unrealized Losses	(220)	(50)
Fair Value	74,648	60,676
Agency bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		28,954
Unrealized Gains		2
Unrealized Losses		(26)
Fair Value		28,930
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	189,310	157,829
Unrealized Gains	0	17
Unrealized Losses	(430)	(160)
Fair Value	188,880	157,686
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		10,473
Unrealized Gains		0
Unrealized Losses		0
Fair Value		10,473
Cash Equivalent
Schedule of Available-for-sale Securities [Line Items]
Fair Value	368,859	48,136
Cash Equivalent | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	358,859	48,136
Cash Equivalent | Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Fair Value	10,000
Cash Equivalent | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Cash Equivalent | Agency bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value		0
Cash Equivalent | U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Cash Equivalent | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value		0
Investments
Schedule of Available-for-sale Securities [Line Items]
Fair Value	263,528	257,765
Investments | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Investments | Certificate of deposit
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0
Investments | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	74,648	60,676
Investments | Agency bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value		28,930
Investments | U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 188,880	157,686
Investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value		$ 10,473

Investments - Strategic Investm

Investments - Strategic Investments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Strategic investments acquired	$ 1,500,000	$ 600,000
Aggregate strategic investments	3,000,000	2,100,000
Impairment losses recognized	$ 600,000	$ 0

Goodwill and Intangible Asset56

Goodwill and Intangible Assets - Schedule of Finite-lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 31,638	$ 31,638
Accumulated Amortization	(31,638)	(24,217)
Net Carrying Amount	0	7,421
Developed technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	29,984	29,984
Accumulated Amortization	(29,984)	(22,711)
Net Carrying Amount	0	7,273
Software license rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,654	1,654
Accumulated Amortization	(1,654)	(1,506)
Net Carrying Amount	$ 0	$ 148

Goodwill and Intangible Asset57

Goodwill and Intangible Assets - Additional Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense									$ 7,400,000	$ 9,300,000	$ 10,600,000
Amortization of certain acquired intangible assets	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 150,000	0	150,000	600,000
Impairment charges									0	0	$ 0
Change in carrying amount of goodwill									$ 0	$ 0

Other Balance Sheet Amounts - P

Other Balance Sheet Amounts - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 52,797	$ 46,699
Less: accumulated depreciation and amortization	(31,980)	(22,737)
Total property and equipment, net	20,817	23,962
Depreciation expense	$ 10,300	9,900	$ 7,600
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	7 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 38,838	32,926
Computer equipment and software | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Computer equipment and software | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,855	3,837
Property and equipment, useful life	7 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 10,046	9,878
Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	6 years
Renovation in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 58	$ 58

Other Balance Sheet Amounts - A

Other Balance Sheet Amounts - Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Accrued bonuses	$ 13,557	$ 13,371
Accrued commissions	12,401	10,616
Other accrued expenses	31,570	23,632
Total accrued expenses	$ 57,528	$ 47,619

Fair Value of Financial Instr60

Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 632,387	$ 305,901
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	74,648	60,676
Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		28,930
U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	188,880	157,686
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	368,859	48,136
Assets and liabilities measured at fair value, total	632,387	305,901
Recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	358,859	48,136
Assets and liabilities measured at fair value, total	358,859	48,136
Recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	10,000	0
Assets and liabilities measured at fair value, total	273,528	257,765
Recurring basis | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Assets and liabilities measured at fair value, total	0	0
Recurring basis | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	74,648	60,676
Recurring basis | Corporate bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	74,648	60,676
Recurring basis | Corporate bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	28,930
Recurring basis | Agency bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | Agency bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	28,930
Recurring basis | Agency bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	188,880	157,686
Recurring basis | U.S. treasury securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | U.S. treasury securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	188,880	157,686
Recurring basis | U.S. treasury securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	10,473
Recurring basis | Commercial paper | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Recurring basis | Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	10,473
Recurring basis | Commercial paper | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 0	$ 0

Fair Value of Financial Instr61

Fair Value of Financial Instruments - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of convertible notes	$ 552,000
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	368,859	$ 48,136
Recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	358,859	48,136
Recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 10,000	$ 0

Debt and Other Financing Arra62

Debt and Other Financing Arrangements - Additional Information (Details)	Jun. 17, 2013USD ($)trading_dayd$ / sharesshares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Debt Instrument [Line Items]
Proceeds from issuance of convertible notes, net			$ 285,077,000	$ 0	$ 0
Proceeds from the issuance of warrants	$ 23,200,000
Number of shares purchasable upon warrant exercise (in shares) | shares	4,700,000
Warrants exercise price (usd per share) | $ / shares	$ 80.06
Convertible Debt
Debt Instrument [Line Items]
Debt recorded		$ 542,810,000	542,810,000	240,450,000
Unamortized debt issuance costs		9,617,000	9,617,000	2,015,000
Unamortized debt discount		$ (10,190,000)	$ (10,190,000)	$ (12,550,000)
The 2018 Notes | Convertible Debt
Debt Instrument [Line Items]
Debt issued, senior convertible notes	$ 253,000,000
Interest rate on convertible notes issued	1.50%
Conversion rate on convertible debt	18.5046
Conversion price on convertible debt (usd per share) | $ / shares	$ 54.04
Debt recorded	$ 214,300,000
Adjustments for equity component of convertible debt	(38,700,000)
Unamortized debt issuance costs	7,300,000
Unamortized debt issuance expense	6,200,000
Issuance cost allocated to equity components	1,100,000
Effective interest rate		5.40%	5.40%
Proceeds from issuance of convertible notes, net	246,000,000
Payments for convertible note hedges	$ (49,500,000)
Note hedges on common stock (in shares) | shares	4,700,000
The 2018 Notes | Convertible Debt | Prior to April 1, 2018 | Maximum
Debt Instrument [Line Items]
Trading day threshold on convertible debt | trading_day	20
Consecutive trading day threshold on convertible debt | trading_day	30
Stock price threshold, convertible debt	130.00%
The 2018 Notes | Convertible Debt | Prior to April 1, 2018 | Minimum
Debt Instrument [Line Items]
Trading day threshold on convertible debt | d	5
Consecutive trading day threshold on convertible debt | trading_day	5
Stock price threshold, convertible debt	98.00%
The 2021 Notes | Convertible Debt
Debt Instrument [Line Items]
Debt issued, senior convertible notes		$ 300,000,000	$ 300,000,000
Interest rate on convertible notes issued		5.75%	5.75%
Conversion rate on convertible debt		23.8095
Conversion price on convertible debt (usd per share) | $ / shares		$ 42	$ 42
Unamortized debt issuance costs		$ 9,100,000	$ 9,100,000
Debt issuance discount		98.00%
Proceeds from issuance of debt		$ 294,000,000
Unamortized debt discount		$ (6,000,000)	$ (6,000,000)
Effective interest rate		6.40%	6.40%
Proceeds from issuance of convertible notes, net		$ 285,100,000

Debt and Other Financing Arra63

Debt and Other Financing Arrangements - Summary of the Net Carrying Amount of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Net carrying value	$ 285,168	$ 238,435
Convertible Debt
Debt Instrument [Line Items]
Principal amount	553,000	253,000
Unamortized debt discount	(10,190)	(12,550)
Net carrying amount before unamortized debt issuance costs	542,810	240,450
Unamortized debt issuance costs	(9,617)	(2,015)
Net carrying value	$ 533,193	$ 238,435

Debt and Other Financing Arra64

Debt and Other Financing Arrangements - Schedule of Note Interest Expense (Details) - Convertible Debt - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 17, 2013
Debt Instrument [Line Items]
Contractual interest expense at 1.50% and 5.75% per annum	$ 4,897	$ 3,795	$ 3,795
Amortization of debt issuance costs	1,472	1,263	1,202
Accretion of debt discount	8,360	7,867	7,489
Total	$ 14,729	$ 12,925	$ 12,486
The 2018 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued				1.50%
The 2021 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued	5.75%

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Nov. 30, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
Common stock authorized (in shares)	1,000,000,000		1,000,000,000
Common stock par value (usd per share)	$ 0.0001		$ 0.0001
Preferred stock authorized (in shares)	50,000,000
Preferred stock, par value (usd per share)	$ 0.0001
Preferred stock issued (in shares)	0		0
Preferred stock outstanding (in shares)	0		0
Share repurchase program, shares authorized to repurchase		$ 100,000,000
Common stock repurchased (in shares)	635,000
Average repurchase price (usd per share)	$ 35.55
Value of shares repurchased	$ 22,599,000
Available amount remaining under share repurchase program	$ 77,400,000

Stock-Based Awards - Plans Info

Stock-Based Awards - Plans Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of the fair market value at time of grant	85.00%
Percentage of the fair market value at the time of exercise	85.00%
Compensation expense recognized	$ 65,830	$ 54,699	$ 43,081
Incentive stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense recognized	$ 14,000	23,000	28,000
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	1,938,000
Compensation expense recognized	$ 44,100	31,900	15,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issuable under plan (in shares)	3,045,181
Compensation expense recognized	$ 1,600	$ 1,400	$ 900
1999 and 2009 Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issuable under plan (in shares)	0
2010 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issuable under plan (in shares)	4,128,903
Additional issuable shares authorization, annual increase, maximum number of shares (in shares)	5,500,000
Additional issuable shares authorization, annual increase, percentage of outstanding stock, maximum	4.50%
2010 Plan | Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	1,937,900
2010 Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under plan (in shares)	3,680,480
2010 Plan | Minimum | Incentive stock options | Individual owning more than 10% of outstanding stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price as percentage of fair market value per share	110.00%
2010 Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares to be added from 1999 and 2009 Plans (in shares)	5,614,369
2010 Plan | Maximum | Incentive stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
2010 Plan | Maximum | Incentive stock options | Individual owning more than 10% of outstanding stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	5 years

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stock Options Outstanding, Shares
Shares outstanding, beginning of period (in shares)	6,041
Granted (in shares)	0
Exercised (in shares)	(413)
Forfeited (in shares)	(334)
Shares outstanding, period end (in shares)	5,294	6,041
Stock Options Outstanding, Weighted Average Exercise Price
Weighted average exercise price, outstanding, beginning of period (in usd per share)	$ 32.01
Weighted average exercise price, granted (in usd per share)	0
Weighted average exercise price, exercised (in usd per share)	16.38
Weighted average exercise price, forfeited (in usd per share)	35.71
Weighted average exercise price, outstanding, period end (in usd per share)	$ 32.99	$ 32.01
Stock Options, Additional Disclosures
Weighted average remaining contractual term, outstanding	5 years 4 months 5 days	6 years 2 months 12 days
Aggregate intrinsic value, outstanding	$ 40,122	$ 74,989
Exercisable (in shares)	4,934
Weighted average exercise price, exercisable (in usd per share)	$ 32.60
Weighted average remaining contractual term, exercisable	5 years 2 months 19 days
Aggregate intrinsic value, exercisable	$ 39,951
Stock Options Vested and Expected to Vest
Shares vested and expected to vest (in shares)	5,280
Weighted average exercise price, vested and expected (in usd per share)	$ 32.98
Weighted average remaining contractual term, vested and expected to vest	5 years 4 months 3 days
Aggregate intrinsic value, vested and expected to vest	$ 40,113
Closing stock price (usd per share)	$ 35.33	$ 42.31

Stock-Based Awards - Schedule o

Stock-Based Awards - Schedule of Stock Options Outstanding and Exercisable (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, number of options (in shares)	5,294
Options outstanding, weighted average remaining contractual life	5 years 4 months 5 days
Options exercisable, number of options (in shares)	4,934
Options exercisable, weighted average remaining contractual life	5 years 2 months 19 days
$0.34 to $1.65
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 0.34
Range of exercise prices, upper (in usd per share) | $ / shares	$ 1.65
Options outstanding, number of options (in shares)	92
Options outstanding, weighted average remaining contractual life	1 year 11 months 12 days
Options exercisable, number of options (in shares)	92
Options exercisable, weighted average remaining contractual life	1 year 11 months 12 days
$5.93 to $8.88
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 5.93
Range of exercise prices, upper (in usd per share) | $ / shares	$ 8.88
Options outstanding, number of options (in shares)	677
Options outstanding, weighted average remaining contractual life	2 years 10 months 12 days
Options exercisable, number of options (in shares)	677
Options exercisable, weighted average remaining contractual life	2 years 10 months 12 days
$12.54 to $15.41
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 12.54
Range of exercise prices, upper (in usd per share) | $ / shares	$ 15.41
Options outstanding, number of options (in shares)	132
Options outstanding, weighted average remaining contractual life	3 years 7 months 27 days
Options exercisable, number of options (in shares)	132
Options exercisable, weighted average remaining contractual life	3 years 7 months 27 days
$16.24 to $18.82
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 16.24
Range of exercise prices, upper (in usd per share) | $ / shares	$ 18.82
Options outstanding, number of options (in shares)	305
Options outstanding, weighted average remaining contractual life	3 years 11 months 29 days
Options exercisable, number of options (in shares)	305
Options exercisable, weighted average remaining contractual life	3 years 11 months 29 days
$20.85 to $23.94
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 20.85
Range of exercise prices, upper (in usd per share) | $ / shares	$ 23.94
Options outstanding, number of options (in shares)	552
Options outstanding, weighted average remaining contractual life	4 years 4 months 15 days
Options exercisable, number of options (in shares)	552
Options exercisable, weighted average remaining contractual life	4 years 4 months 15 days
$27.55 to $31.44
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 27.55
Range of exercise prices, upper (in usd per share) | $ / shares	$ 31.44
Options outstanding, number of options (in shares)	207
Options outstanding, weighted average remaining contractual life	5 years 6 months 21 days
Options exercisable, number of options (in shares)	193
Options exercisable, weighted average remaining contractual life	5 years 5 months 16 days
$31.64 to $36.15
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 31.64
Range of exercise prices, upper (in usd per share) | $ / shares	$ 36.15
Options outstanding, number of options (in shares)	835
Options outstanding, weighted average remaining contractual life	6 years 10 months 1 day
Options exercisable, number of options (in shares)	633
Options exercisable, weighted average remaining contractual life	6 years 7 months 7 days
$38.03 to $45.76
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 38.03
Range of exercise prices, upper (in usd per share) | $ / shares	$ 45.76
Options outstanding, number of options (in shares)	1,093
Options outstanding, weighted average remaining contractual life	6 years 27 days
Options exercisable, number of options (in shares)	1,036
Options exercisable, weighted average remaining contractual life	6 years 17 days
$46.20 to $56.05
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices, lower (in usd per share) | $ / shares	$ 46.20
Range of exercise prices, upper (in usd per share) | $ / shares	$ 56.05
Options outstanding, number of options (in shares)	1,401
Options outstanding, weighted average remaining contractual life	6 years 1 month 13 days
Options exercisable, number of options (in shares)	1,314
Options exercisable, weighted average remaining contractual life	6 years 1 month 5 days

Stock-Based Awards - Stock Opti

Stock-Based Awards - Stock Option Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense recognized	$ 65,830	$ 54,699	$ 43,081
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of options exercised	9,200	18,200	11,400
Total grant date fair value of stock options vested	15,400	24,300	32,300
Compensation expense recognized	14,000	23,000	28,000
Unrecognized compensation expense related to options	$ 5,400
Unrecognized compensation expense, weighted-average period of recognition	1 year
Aggregate grant date fair value of stock options granted		$ 1,800	$ 8,400

Stock-Based Awards - Restricted

Stock-Based Awards - Restricted Stock Units (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Additional Information
Compensation expense recognized	$ 65,830	$ 54,699	$ 43,081
Restricted stock units
Number of Shares
Number of shares, outstanding at beginning of period (in shares)	3,254
Number of shares, granted (in shares)	1,938
Number of shares, forfeited (in shares)	(367)
Number of shares, vested (in shares)	(1,035)
Number of shares, outstanding at period end (in share)	3,790	3,254
Weighted Average Grant Date Fair Value
Weighted average grant date fair value, outstanding at beginning of period (in usd per share)	$ 36.35
Weighted average grant date fair value, granted (in usd per share)	37.99
Weighted average grant date fair value, forfeited (in usd per share)	36.04
Weighted average grant date fair value, vested (in usd per share)	36.34
Weighted average grant date fair value, outstanding at period end (in usd per share)	$ 37.22	$ 36.35
Additional Information
Compensation expense recognized	$ 44,100	$ 31,900	$ 15,000
Unrecognized compensation expense related to nonvested restricted stock units	$ 105,700
Unrecognized compensation expense, weighted-average period of recognition		2 years 8 months 23 days

Stock-Based Awards - Performanc

Stock-Based Awards - Performance Awards Information (Details) - USD ($)	1 Months Ended					12 Months Ended
	Mar. 31, 2017	Jul. 31, 2016	Feb. 29, 2016	Feb. 28, 2015	Jul. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense						$ 65,830,000	$ 54,699,000	$ 43,081,000
Performance awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years	3 years
Share-based compensation expense						11,200,000	$ 2,600,000	$ 1,900,000
Unrecognized compensation expense						0
Performance based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years		3 years
Expected to vest (in shares)				535,000	40,600
Aggregated grant date fair value					$ 1,800,000
Performance based restricted stock units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected to vest (in shares)				1,070,000	60,900
Tranche One | Performance based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregated grant date fair value				$ 9,900,000		4,800,000
Tranche Two | Performance based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregated grant date fair value			$ 3,600,000			$ 9,800,000

Stock-Based Awards - Issuance o

Stock-Based Awards - Issuance of Awards Under New Compensation Award Structure (Details) - Performance awards - $ / shares	1 Months Ended
	Mar. 31, 2017	Jul. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years	3 years
Weighted average grant date fair value (in usd per share)	$ 41.73	$ 38.67
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	185,270	166,600
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted (in shares)	555,810	499,800

Stock-Based Awards - Schedule73

Stock-Based Awards - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 65,830	$ 54,699	$ 43,081
Cost of revenue
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	4,904	4,732	3,887
Sales and marketing
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	28,427	25,642	23,604
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	9,630	7,586	6,010
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based compensation expense	$ 22,869	$ 16,739	$ 9,580

Income Taxes - Components of In

Income Taxes - Components of Income Before Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States									$ (32,853)	$ (39,107)	$ (59,797)
Foreign									(26,736)	(26,523)	(24,538)
Loss before income tax provision	$ (8,696)	$ (14,352)	$ (20,901)	$ (15,640)	$ (18,305)	$ (12,170)	$ (17,447)	$ (17,708)	$ (59,589)	$ (65,630)	$ (84,335)

Income Taxes - Components of 75

Income Taxes - Components of Income Tax Provision (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax provision:
Federal									$ 0	$ 0	$ 0
State									114	105	147
Foreign									1,580	1,838	1,139
Total current income tax provision									1,694	1,943	1,286
Deferred income tax benefit:
Federal									0	0	0
State									0	0	0
Foreign									52	(736)	(105)
Total deferred income tax benefit									52	(736)	(105)
Total income tax provision (benefit)	$ 308	$ 503	$ 364	$ 571	$ 313	$ 218	$ 141	$ 535	$ 1,746	$ 1,207	$ 1,181

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	Jan. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Net increase to valuation allowance primarily due to additional net operating losses		$ 6,800,000	$ 15,500,000	$ 22,400,000
Previously unrecognized excess tax benefit	$ 39,400,000		39,400,000
Undistributed foreign earnings		3,300,000	4,000,000
Potential tax impact if undistributed foreign earnings were distributed		100,000	$ 800,000
Transition tax on undistributed earnings		0
Reduction in tax expense if unrecognized tax benefits are recognized		1,300,000
Decrease in unrecognized tax benefits and income tax due to resolution		1,100,000
Federal Tax Authority
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses		250,700,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses		257,900,000
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Federal, state and foreign net operating losses		$ 86,000,000

Income Taxes - Components of Ef

Income Taxes - Components of Effective Income Tax Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation
U.S. Federal tax benefit at statutory rates									$ (20,260)	$ (22,310)	$ (28,681)
State income taxes, net of federal tax benefit									(806)	(855)	(1,632)
Foreign rate differential									5,220	3,711	3,964
Stock based compensation									3,182	4,467	4,673
Other permanent differences									(494)	(750)	(99)
Deferred adjustments / U.S. rate change									7,811	0	0
Other									262	1,494	536
Valuation allowance									6,831	15,450	22,420
Total income tax provision (benefit)	$ 308	$ 503	$ 364	$ 571	$ 313	$ 218	$ 141	$ 535	$ 1,746	$ 1,207	$ 1,181

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued expenses	$ 2,371	$ 3,037
Long-lived intangible assets and fixed assets — basis difference	19,884	22,146
Net operating loss carryforwards	80,615	68,356
Stock-based compensation	16,886	19,515
Deferred revenue	2,739	4,060
Convertible note hedge	1,467	6,387
Other	2,721	2,276
Total deferred tax assets	126,683	125,777
Valuation allowance	(118,606)	(111,775)
Deferred tax assets, net of valuation allowance	8,077	14,002
Deferred tax liabilities:
Prepaid expenses and deferred commissions	(5,672)	(7,718)
Convertible note discount	(1,074)	(4,721)
Other	(410)	(591)
Total deferred tax liabilities	(7,156)	(13,030)
Net deferred tax assets (liabilities)	$ 921	$ 972

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits
Unrecognized tax benefit, beginning balance	$ 276	$ 276	$ 276
Additions for tax positions related to the current year	995	0	0
Unrecognized tax benefit, ending balance	$ 1,271	$ 276	$ 276

Restructuring Costs (Details)

Restructuring Costs (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring and Related Activities [Abstract]
Restructuring costs	$ 1,539	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 1,539	$ 0	$ 0
Expected cost of expenditures	$ 3,000								$ 3,000

Segment and Geographic Inform81

Segment and Geographic Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment									1
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 131,956	$ 121,796	$ 116,651	$ 111,582	$ 109,029	$ 107,758	$ 107,013	$ 99,324	$ 481,985	$ 423,124	$ 339,651
Property and equipment, net	20,817				23,962				20,817	23,962
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									313,729	284,657	228,724
Property and equipment, net	16,468				19,843				16,468	19,843
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									25,701	27,571	30,104
Property and equipment, net	3,378				2,985				3,378	2,985
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									142,555	110,896	80,823
Property and equipment, net	$ 971				$ 1,134				971	1,134
Subscription revenue | Sales Revenue, Services, Net | Product Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 396,764	$ 339,756	$ 270,093
Percentage of revenue to total									82.30%	80.30%	79.50%
Professional services revenue | Sales Revenue, Services, Net | Product Concentration Risk
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 85,221	$ 83,368	$ 69,558
Percentage of revenue to total									17.70%	19.70%	20.50%

401(k) Savings Plan (Details)

401(k) Savings Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
401(k) employer matching percentage	50.00%
Maximum 401(k) annual contributions by employer per employee	$ 2,400
401(k) vesting period	4 years
401(k) matching contribution expenses	$ 2,000,000	$ 1,900,000	$ 1,600,000

Commitments and Contingencies -

Commitments and Contingencies - Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Rent expense under operating leases	$ 8,000	$ 7,800	$ 6,700
Building lease and other commitments
Operating Leases
2,018	8,246
2,019	826
2,020	84
2,021	73
2,022	0
Total minimum lease payments	$ 9,229

Commitments and Contingencies84

Commitments and Contingencies - Letters of Credit (Details) $ in Millions	Dec. 31, 2017USD ($)
Standby letters of credit | Building lease and other commitments
Loss Contingencies [Line Items]
Letter of credit outstanding	$ 1.4

Commitments and Contingencies85

Commitments and Contingencies - Additional Information (Detail) $ in Millions	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Contractual commitment due within the next year	$ 12
Contractual commitment due within two years	6.5
Contractual commitment due within three years	0.7
Software Subscription Agreement
Loss Contingencies [Line Items]
Contractual commitment due within the next year	8.4
Contractual commitment due within two years	5.3
Contractual commitment due within three years	0.5
Sponsorship Agreement With Professional Sports Franchise
Loss Contingencies [Line Items]
Contractual commitment due within the next year	0.7
Contractual commitment due within two years	$ 0.7

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Value of resources donated to related parties	$ 3.4	$ 3.3	$ 2.9
Director
Related Party Transaction [Line Items]
Expenses from transactions with related party		$ 0.7	$ 0.6
Expenses related to products and services provided by related party	0.7
Revenue from related party	$ 0.3

Subsequent Events (Detail)

Subsequent Events (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		2 Months Ended	12 Months Ended
	Feb. 09, 2018	Jan. 31, 2018	Feb. 27, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Contractual commitment due within the next year				$ 12,000
Contractual commitment due within two years				6,500
Contractual commitment due within three years				$ 700
Common stock repurchased (in shares)				635,000
Average repurchase price (usd per share)				$ 35.55
Value of shares repurchased				$ 22,599
Restricted stock units
Subsequent Event [Line Items]
Awards granted (in shares)				1,938,000
Software Subscription Agreement
Subsequent Event [Line Items]
Contractual commitment due within the next year				$ 8,400
Contractual commitment due within two years				5,300
Contractual commitment due within three years				$ 500
2010 Plan | Restricted stock units
Subsequent Event [Line Items]
Awards granted (in shares)				1,937,900
Subsequent Event
Subsequent Event [Line Items]
Common stock repurchased (in shares)	300,000
Average repurchase price (usd per share)	$ 37.03
Value of shares repurchased	$ 9,900
Subsequent Event | Restricted stock units
Subsequent Event [Line Items]
Awards granted (in shares)			64,635
Award vesting period			4 years
Subsequent Event | Software Subscription Agreement
Subsequent Event [Line Items]
Contractual commitment due within the next year			$ 3,100
Contractual commitment due within two years			3,100
Contractual commitment due within three years			$ 3,100
Subsequent Event | Employee Stock Purchase Plan
Subsequent Event [Line Items]
Share increase under plan (in shares)		575,119
Subsequent Event | 2010 Plan
Subsequent Event [Line Items]
Share increase under plan (in shares)		2,588,036

Selected Quarterly Data (Unau88

Selected Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unaudited Quarterly Consolidated Statements of Operations
Revenue	$ 131,956	$ 121,796	$ 116,651	$ 111,582	$ 109,029	$ 107,758	$ 107,013	$ 99,324	$ 481,985	$ 423,124	$ 339,651
Cost of revenue	37,889	35,708	35,321	33,949	34,778	33,369	35,955	31,650	142,867	135,752	109,864
Gross profit	94,067	86,088	81,330	77,633	74,251	74,389	71,058	67,674	339,118	287,372	229,787
Operating expenses:
Sales and marketing	60,750	60,554	62,073	56,894	57,405	53,690	57,835	56,701	240,271	225,631	207,026
Research and development	17,491	16,389	14,684	13,411	12,050	12,130	11,782	11,015	61,975	46,977	40,991
General and administrative	19,723	21,249	23,141	20,476	19,345	18,608	16,538	16,465	84,589	70,956	49,877
Restructuring	1,539	0	0	0	0	0	0	0	1,539	0	0
Amortization of certain acquired intangible assets	0	0	0	0	0	0	0	150	0	150	600
Total operating expenses	99,503	98,192	99,898	90,781	88,800	84,428	86,155	84,331	388,374	343,714	298,494
Loss from operations	(5,436)	(12,104)	(18,568)	(13,148)	(14,549)	(10,039)	(15,097)	(16,657)	(49,256)	(56,342)	(68,707)
Other income (expense):
Interest income (expense) and other income (expense), net	(3,260)	(2,248)	(2,333)	(2,492)	(3,756)	(2,131)	(2,350)	(1,051)	(10,333)	(9,288)	(15,628)
Loss before income tax provision	(8,696)	(14,352)	(20,901)	(15,640)	(18,305)	(12,170)	(17,447)	(17,708)	(59,589)	(65,630)	(84,335)
Income tax provision	(308)	(503)	(364)	(571)	(313)	(218)	(141)	(535)	(1,746)	(1,207)	(1,181)
Net loss	$ (9,004)	$ (14,855)	$ (21,265)	$ (16,211)	$ (18,618)	$ (12,388)	$ (17,588)	$ (18,243)	$ (61,335)	$ (66,837)	$ (85,516)
Net loss per share — basic and diluted (usd per share)	$ (0.16)	$ (0.26)	$ (0.37)	$ (0.29)	$ (0.33)	$ (0.22)	$ (0.32)	$ (0.33)	$ (1.07)	$ (1.20)	$ (1.58)
Weighted average common shares outstanding, basic and diluted (in shares)	57,826	57,627	56,935	56,642	56,300	55,964	55,278	54,827	57,262	55,595	54,171