The Trade Desk, Inc. (TTD) 10-K 22 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	TTD
Entity Registrant Name	TRADE DESK, INC.
Entity Central Index Key	1,671,933
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Public Float			$ 3,279,011,436
Class A common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		37,544,005
Class B common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		6,529,619

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 207,232	$ 155,950
Accounts receivable, net	834,764	599,565
Prepaid expenses and other current assets	14,527	10,298
TOTAL CURRENT ASSETS	1,056,523	765,813
Property and equipment, net	33,046	17,405
Deferred income taxes	8,460	3,359
Other assets, non-current	19,843	10,587
TOTAL ASSETS	1,117,872	797,164
Current liabilities:
Accounts payable	669,147	490,377
Accrued expenses and other current liabilities	44,844	28,155
TOTAL CURRENT LIABILITIES	713,991	518,532
Debt, net		27,000
Other liabilities, non-current	9,314	6,049
TOTAL LIABILITIES	723,305	551,581
Commitments and contingencies (Note 12)
STOCKHOLDERS’ EQUITY
Preferred stock, par value $0.000001; 100,000 shares authorized, zero shares issued and outstanding as of December 31, 2018 and 2017
Common stock, par value $0.000001; 1,000,000 Class A shares authorized as of December 31, 2018 and 2017; 36,822 and 32,486 shares issued and outstanding as of December 31, 2018 and 2017, respectively; 95,000 Class B shares authorized as of December 31, 2018 and 2017; 7,042 and 9,155 shares issued and outstanding as of December 31, 2018 and 2017, respectively	0	0
Additional paid-in capital	270,447	209,603
Retained earnings	124,120	35,980
TOTAL STOCKHOLDERS’ EQUITY	394,567	245,583
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 1,117,872	$ 797,164

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Preferred stock
Preferred stock, par value (in dollars per share)	$ 0.000001	$ 0.000001
Preferred stock, authorized shares	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock
Common stock, par value (in dollars per share)	$ 0.000001	$ 0.000001
Common stock, authorized shares	1,095,000,000
Class A common stock
Common stock
Common stock, authorized shares	1,000,000,000	1,000,000,000
Common stock, shares issued	36,822,000	32,486,000
Common stock, shares outstanding	36,822,000	32,486,000
Class B common stock
Common stock
Common stock, authorized shares	95,000,000	95,000,000
Common stock, shares issued	7,042,000	9,155,000
Common stock, shares outstanding	7,042,000	9,155,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 477,294	$ 308,217	$ 202,926
Type of Revenue [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Operating expenses:
Platform operations	$ 114,098	$ 66,230	$ 39,876
Type of Cost, Good or Service [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Sales and marketing	$ 87,071	$ 61,379	$ 46,056
Technology and development	83,892	52,806	27,313
General and administrative	84,910	58,446	32,163
Total operating expenses	369,971	238,861	145,408
Income from operations	107,323	69,356	57,518
Other expense (income):
Interest expense	1,550	1,791	3,075
Interest income	(1,883)	(93)
Change in fair value of preferred stock warrant liabilities			9,458
Foreign currency exchange loss, net	1,919	4,033	1,151
Total other expense, net	1,586	5,731	13,684
Income before income taxes	105,737	63,625	43,834
Provision for income taxes	17,597	12,827	23,352
Net income	88,140	50,798	20,482
Net income (loss) attributable to common stockholders	$ 88,140	$ 50,798	$ (26,727)
Net income (loss) per share attributable to common stockholders:
Basic	$ 2.08	$ 1.26	$ (1.46)
Diluted	$ 1.92	$ 1.15	$ (1.46)
Weighted average shares outstanding:
Basic	42,442	40,262	18,280
Diluted	45,793	44,056	18,280

CONSOLIDATED STATEMENT OF CONVE

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands		Total	Convertible Preferred Stock	Common Stock		Common StockIPO	Additional Paid-In Capital	Retained Earnings (Accumulated Deficit)
Balance at beginning of period at Dec. 31, 2015			$ 24,204
Balance at beginning of period (in shares) at Dec. 31, 2015			66,330	10,884	[1]
Balance at beginning of period at Dec. 31, 2015		$ 14,142					$ 1,039	$ 13,103
Issuance of series C convertible preferred stock, net of issuance costs			$ 59,871
Issuance of series C convertible preferred stock, net of issuance costs (in shares)			11,501
Reclassification of preferred stock warrant liability upon net exercise of warrant			$ 3,789
Reclassification of preferred stock warrant liability upon net exercise of warrant (in shares)			789
Repurchase of convertible preferred stock		(47,209)	$ (4,623)				(1,168)	(46,041)
Repurchase of convertible preferred stock (in shares)			(12,384)
Repurchase and retirement of common stock		(2,362)						(2,362)
Repurchase and retirement of common stock (in shares)	[1]			(189)
Issuance of common stock to employee		73,634					73,634
Issuance of common stock to employee (in shares)	[1]					4,667
Conversion of convertible preferred stock to Class B common stock in connection with IPO		83,241					83,241
Conversion of convertible preferred stock to Class B common stock in connection with IPO (in shares)			(66,236)	22,079	[1]
Conversion of convertible preferred stock to Class B common stock in connection with IPO			$ (83,241)
Conversion of warrant for convertible preferred stock to a warrant for Class B common stock in connection with IPO		12,596					12,596
Net exercise of warrant to purchase Class B common stock (in shares)	[1]			449
Exercise of common stock options		488					488
Exercise of common stock options (in shares)	[1]			785
Stock-based compensation		5,144					5,144
Issuance of common stock under employee stock purchase plan		4,224					4,224
Issuance of common stock under employee stock purchase plan (in shares)	[1]			276
Grants of restricted stock (in shares)	[1]			180
Net income		20,482						20,482
Balance at end of period (in shares) at Dec. 31, 2016	[1]			39,131
Balance at end of period at Dec. 31, 2016		164,380					179,198	(14,818)
Exercise of common stock options		2,565					2,565
Exercise of common stock options (in shares)	[1]			1,932
Stock-based compensation		21,860					21,860
Issuance of common stock under employee stock purchase plan		6,997					6,997
Issuance of common stock under employee stock purchase plan (in shares)	[1]			433
Restricted stock, net of forfeitures and shares withheld for taxes		(1,017)					(1,017)
Restricted stock, net of forfeitures and shares withheld for taxes (in shares)	[1]			145
Net income		50,798						50,798
Balance at end of period (in shares) at Dec. 31, 2017	[1]			41,641
Balance at end of period at Dec. 31, 2017		245,583					209,603	35,980
Exercise of common stock options		10,021					10,021
Exercise of common stock options (in shares)	[1]			1,446
Stock-based compensation		43,695					43,695
Issuance of common stock under employee stock purchase plan		13,805					13,805
Issuance of common stock under employee stock purchase plan (in shares)	[1]			699
Restricted stock, net of forfeitures and shares withheld for taxes		(6,677)					(6,677)
Restricted stock, net of forfeitures and shares withheld for taxes (in shares)	[1]			78
Net income		88,140						88,140
Balance at end of period (in shares) at Dec. 31, 2018	[1]			43,864
Balance at end of period at Dec. 31, 2018		$ 394,567					$ 270,447	$ 124,120
[1]	Refer to Note 8-Capitalization for discussion of the establishment of the Company’s two classes of common stock and the reclassification of its common stock into Class B common stock prior to the Company’s initial public offering (“IPO”) in September 2016.

CONSOLIDATED STATEMENT OF CON_2

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)Class
Number of classes of common stock | Class	2
Common Stock
Underwriters' commissions and offering costs | $	$ 10,366

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES:
Net income	$ 88,140	$ 50,798	$ 20,482
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	11,822	7,209	3,798
Stock-based compensation	42,210	21,317	5,056
Change in fair value of preferred stock warrant liabilities			9,458
Deferred income taxes	(5,101)	(1,581)	(607)
Bad debt expense	2,115	4,289	1,890
Other	2,905	(1,303)	1,160
Changes in operating assets and liabilities:
Accounts receivable	(239,901)	(224,636)	(187,736)
Prepaid expenses and other assets	(10,551)	(5,033)	(2,675)
Accounts payable	177,675	171,793	209,483
Accrued expenses and other liabilities	17,289	8,371	14,722
Net cash provided by operating activities	86,603	31,224	75,031
INVESTING ACTIVITIES:
Purchases of property and equipment	(19,795)	(10,110)	(6,884)
Capitalized software development costs	(5,396)	(2,954)	(2,337)
Business acquisition		(3,000)
Net cash used in investing activities	(25,191)	(16,064)	(9,221)
FINANCING ACTIVITIES:
Repayment on line of credit	(27,000)		(65,000)
Repayment of term debt			(30,000)
Proceeds from line of credit			75,847
Payment of debt financing costs	(279)	(154)	(976)
Payment of financing obligations		(1,001)	(550)
Proceeds from exercise of stock options	10,021	2,565	488
Proceeds from employee stock purchase plan	13,805	6,997	4,224
Taxes paid related to net settlement of restricted stock awards	(6,677)	(1,017)
Proceeds from issuance of Series C convertible preferred stock			60,000
Repurchase of preferred stock and common stock			(54,000)
Payment of stock repurchase costs			(155)
Payment of Series C convertible preferred stock offering cost			(129)
Proceeds from the issuance of Class A common stock in initial public offering, net of underwriting commissions			78,120
Payment of offering costs—initial public offering			(4,326)
Net cash provided by (used in) financing activities	(10,130)	7,390	63,543
Increase in cash and cash equivalents	51,282	22,550	129,353
Cash and cash equivalents—Beginning of year	155,950	133,400	4,047
Cash and cash equivalents—End of year	207,232	155,950	133,400
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid for income taxes	17,287	19,163	16,740
Cash paid for interest	817	1,320	1,696
Capitalized assets financed by accounts payable	1,944	701	3,490
Tenant improvements paid by lessor	1,811	640
Stock-based compensation included in capitalized software development costs	1,485	543	88
Asset retirement obligation	$ 907		354
Debt financing costs included in debt, net		$ 1,153
Conversion of convertible preferred stock to Class B common stock			83,241
Conversion of warrant for convertible preferred stock to a warrant for Class B common stock and net exercise of warrant to purchase Class B common stock			12,596
Net exercise of warrants to purchase Series Seed convertible preferred stock			$ 3,789

Nature of Operations

Nature of Operations	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Nature of Operations	Note 1—Nature of Operations The Trade Desk, Inc. (the “Company”) was formed in November 2009 as a Delaware corporation. The Company is headquartered in Ventura, California and has offices in various cities in North America, Europe, Asia and Australia. The Company is a technology company that empowers buyers of advertising by providing a self-service cloud-based platform on which ad buyers can create, manage, and optimize more expressive data-driven digital advertising campaigns across ad formats, including display, video, audio, native and, social, on a multitude of devices, such as computers, mobile devices, and connected TV (CTV).

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	Note 2—Basis of Presentation and Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the operations of the Company and its wholly owned subsidiaries. All intercompany transactions have been eliminated in consolidation. The Company has no components of other comprehensive income (loss), and accordingly, the Company’s comprehensive income is the same as its net income for all periods presented. 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 consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ materially from these estimates. On an on-going basis, management evaluates its estimates, primarily those related to: (1) revenue recognition criteria, including the determination of revenue reporting as net versus gross in the Company’s revenue arrangements, (2) allowances for doubtful accounts, (3) the useful lives of property and equipment and capitalized software development costs, (4) income taxes, (5) assumptions used in the Black-Scholes option pricing model to determine the fair value of stock-based compensation, (6) the recognition and disclosure of contingent liabilities and (7) the assumptions used in valuing acquired assets and assumed liabilities in business combinations Revenue Recognition On January 1, 2018, the Company adopted Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) The Company determines revenue recognition through the following steps: • Identification of a contract with a customer; • Identification of the performance obligations in the contract; • Determination of the transaction price; • Allocation of the transaction price to the performance obligations in the contract; and • Recognition of revenue when or as the performance obligations are satisfied. The Company maintains agreements with each client and supplier in the form of master service agreements, which set out the terms of the relationship and access to the Company’s platform. The Company’s performance obligation is to provide the use of its platform to clients to develop ad campaigns and select the advertising inventory, data and other add-on features. The Company charges clients a platform fee, based on a percentage of a client’s purchases through the platform, and the transaction price is determined based on the consideration to which it expects to be entitled in exchange for the completion of a transaction, that is, when a bid is won. The Company reports revenue net of amounts it pays suppliers for the cost of advertising inventory, third-party data and other add-on features (collectively, “Supplier Features”). The determination of whether the Company is the principal or agent, and hence whether to report revenue on a gross basis for the amount of the Supplier Features the clients purchase using the platform plus the Company’s platform fees or on a net basis for the amount of platform fees charged to the client, requires judgment. The Company determined that it is not primarily responsible for the purchase of Supplier Features, but rather, it is primarily responsible to provide a platform that enables clients to bid on advertising inventory, and use data and other add-on features in designing and executing their campaigns. The Company does not control the Supplier Features prior to the purchase by the client, and it does not have pricing latitude with respect to the cost of such features. The platform fee the Company charges clients is a percentage of their purchases through its platform, similar to a commission, and the platform fee is not contingent on the results of an advertising campaign. Based on these and other factors, the Company determined that it is not the principal in the purchase and sale of Supplier Features in all of its arrangements, and therefore, it reports revenue on a net basis for the platform fees charged to clients. The Company generally bills clients for the gross amount of Supplier Features they purchase through its platform and the platform fees, net of allowances (“Gross Billings”). Some of the Company’s clients have payment relationships directly with advertising inventory suppliers in which case the Company only bills these clients for third-party data, other add-on features and its platform fees. The Company invoices its clients on a monthly basis for the purchases occurring during the month. Invoice payment terms, negotiated on a client-by-client basis, are typically between 30 to 90 days. However, for certain agency clients with sequential liability terms, payments are not due to the Company until such agency client has received payment from its customers who are advertisers. The Company’s accounts receivable are recorded at the amount of Gross Billings for the amounts it is responsible to collect, and accounts payable are recorded at the net amount payable to suppliers. Accordingly, both accounts receivable and accounts payable appear large in relation to revenue reported on a net basis. Refer to Note 11 for geographic information related to Gross Billings. Operating Expenses The Company classifies its operating expenses into four categories: Platform Operations. Platform operations expense consists of expenses related to hosting the Company’s platform, which includes “internet traffic” associated with the viewing of available impressions or queries per second (QPS) and providing support to clients. Platform operations expense includes hosting costs, personnel costs, and amortization of acquired technology and capitalized software costs for the development of the Company’s platform, including allocated overhead. Personnel costs included in platform operations include salaries, bonuses, stock-based compensation, and employee benefit costs, and are primarily attributable to personnel who provide the Company’s clients with support using the Company’s platform and the personnel who support the Company’s platform. The Company capitalizes certain costs associated with the development of the Company’s platform and amortizes these costs over their estimated useful lives in platform operations expense. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Sales and Marketing. Sales and marketing expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation, employee benefits costs and commission costs, for the Company’s sales and marketing personnel. Sales and marketing expense also includes costs for market development programs, advertising, promotional and other marketing activities, and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Commissions costs are expensed as incurred. Technology and Development. The Company’s technology and development expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation and employee benefits costs, third-party consultant costs associated with the ongoing development and maintenance of the Company’s platform and integrations with our advertising and data inventory suppliers, amortization of capitalized third-party software used in the development of the Company’s platform and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Technology and development costs are expensed as incurred, except to the extent that such costs are associated with software development that qualifies for capitalization, which are then recorded as capitalized software development costs included in other assets, non-current on the Company’s consolidated balance sheet. The Company amortizes capitalized software development costs relating to the Company’s platform to platform operations expense. General and Administrative. The Company’s general and administrative expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation, and employee benefits costs associated with the Company’s executive, finance, legal, human resources, compliance, and other administrative personnel, as well as accounting and legal professional services fees, bad debt expense and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Stock-Based Compensation Compensation expense related to stock options, restricted stock awards and units, which are referred to collectively as restricted stock, and awards granted under the Company’s employee stock purchase plan (“ESPP”), is measured and recognized in the consolidated financial statements based on the fair value of the awards granted. The fair value of each option award is estimated on the grant date using the Black-Scholes option-pricing model. The fair value of restricted stock is calculated using the closing market price of the Company’s common stock on the date of grant, and for performance-based restricted stock, we also assess the probability of targets being achieved in determining the amount of expense to recognize. Stock-based compensation expense related to stock options and restricted stock is recognized on a straight-line basis over the requisite service periods of the awards, which is generally four years. For performance-based restricted stock, expense is recognized on a graded-vesting attribution basis over the requisite service period of the award and is adjusted in subsequent reporting periods if the assessed probability or estimated level of achievement of the performance goals changes. Stock-based compensation expense for ESPP awards is recognized on a graded-vesting attribution basis over the requisite service period of each award. Determining the fair value of stock options and ESPP awards requires judgment. The Company’s use of the Black-Scholes option pricing model requires the input of subjective assumptions, including the fair value of the underlying common stock for periods prior to the completion of the Company’s IPO in September 2016, the expected term of the option, the expected volatility of the Company’s common stock, risk-free interest rates, and the expected dividend yield of the Company’s common stock. The assumptions used in the Company’s option-pricing model represent management’s best estimates. These estimates involve inherent uncertainties and the application of management’s judgment. These assumptions and estimates are as follows: Fair Value of Common Stock. For stock options granted subsequent to the Company’s IPO in September 2016 and ESPP awards, the fair value of common stock is based on the closing price of its common stock as reported on the NASDAQ Global Market on the grant date. Prior to the IPO in September 2016, the board of directors determined the fair value of the common stock at the time of the grant of options by considering a number of objective and subjective factors including the Company’s actual operating and financial performance, market conditions and performance of comparable publicly traded companies, developments and milestones in the Company, the likelihood of achieving a liquidity event and transactions involving the Company’s preferred or common stock, among other factors. The fair value was determined in accordance with applicable elements of the practice aid issued by the American Institute of Certified Public Accountants, . Risk The risk-free interest rate is based on the yields of U.S. Treasury securities with maturities approximating the expected term of the awards. Expected Term. Given insufficient historical data relating to stock option exercises, to determine the expected term, the Company applies the 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. For ESPP awards, the expected term is the time period from the grant date to the respective purchase dates included within each offering period. Volatility. Because the Company has a short trading history for its common stock, the Company determines the price volatility based on a blend of the historical volatilities of a publicly traded peer group, implied volatilities from its traded options, and its historical volatility, based on daily price observations over a period equivalent to the expected term of the award. Dividend Yield. The dividend yield assumption is based on the Company’s history and current expectations of dividend payouts. The Company has never declared or paid any cash dividends on its common stock and does not anticipate paying any cash dividends in the foreseeable future, so the Company used an expected dividend yield of zero. During the year ended December 31, 2016, the Company early adopted Accounting Standards Update (“ASU”) No. 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share—Based Payment Accounting The Company will continue to use judgment in evaluating the assumptions related to the Company’s stock-based compensation. Future expense amounts for any particular period could be affected by changes in the Company’s assumptions or market conditions. Income Taxes Deferred income tax assets and liabilities are determined based upon the net tax effects of the differences between the Company’s consolidated financial statements carrying amounts and the tax basis of assets and liabilities and are measured using the enacted tax rate expected to apply to taxable income in the years in which the differences are expected to be reversed. A valuation allowance is used to reduce some or all of the deferred tax assets if, based upon the weight of available evidence, it is more likely than not that those deferred tax assets will not be realized. The Company recognizes the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized. The Company recognizes interest and penalties accrued related to its uncertain tax positions in its income tax provision in the accompanying consolidated statements of operations. In January 2018, the FASB released guidance on the accounting for tax on the global intangible low-taxed income ("GILTI") provisions of the Tax Cuts and Jobs Act (the "Act"). The GILTI provisions impose a tax on foreign income in excess of a deemed return on tangible assets of foreign corporations. The guidance indicates that either accounting for deferred taxes related to GILTI inclusions or to treat any taxes on GILTI inclusions as period cost are both acceptable methods subject to an accounting policy election. Effective the first quarter of 2018, the Company elected to treat any potential GILTI inclusions as a period cost. Net Income (Loss) Per Share Attributable to Common Stockholders Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding. The Company applies the two-class method to allocate net income (loss) between common and other participating securities based on their participation rights. Prior to the conversion of preferred stock to common stock concurrent with the IPO in September 2016 and because the holders of the Company’s convertible preferred stock were entitled to participate in dividends, the Company allocated net income to common and preferred stock based on their respective rights to receive dividends, whether or not declared. For 2016, the excess of the repurchase price of preferred stock over its carrying value has been recorded as a reduction to net income to determine net loss attributable to common stockholders. Diluted net income (loss) per share attributable to common stockholders adjusts the basic net income (loss) per share attributable to common stockholders and the weighted-average number of shares of common stock outstanding for the potentially dilutive impact of stock options, restricted stock, ESPP and warrants, using the treasury-stock method, and convertible preferred stock using the as-if-converted method. Cash, Cash Equivalents and Marketable Securities The Company considers all short-term highly liquid investments with an original maturity of three months or less to be cash equivalents. Cash equivalents, consisting of money market funds and time deposits, are carried at fair value. Refer to Note 6—Fair Value Measurements for additional information regarding the fair value of cash equivalents. Subsequent to December 31, 2018, the Company used existing cash and cash equivalents to purchase $50.9 million in marketable securities classified as short-term investments. The Company classifies its marketable securities as available-for-sale investments in its current assets because they represent investments of cash available for current operations. Available-for-sale investments are carried at fair value with any unrealized gains and losses, net of taxes, included in accumulated other comprehensive income (loss) in stockholders' equity. Unrealized losses are charged against other expense when a decline in fair value is determined to be other-than-temporary. The Company determines realized gains or losses on sale of marketable securities on a specific identification method, and records such gains or losses as other expense (income). Accounts Receivable and Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount, are unsecured and do not bear interest. The Company performs ongoing credit evaluations of its clients and certain advertisers when the Company’s agreements with its clients contain sequential liability terms that provide that the client payments are not due to the Company until the client has received payment from its customers who are advertisers. The allowance for doubtful accounts is based on the best estimate of the amount of probable credit losses in existing accounts receivable. The allowance for doubtful accounts is determined based on historical collection experience and the review in each period of the status of the then-outstanding accounts receivable, while taking into consideration current client information, subsequent collection history and other relevant data. The Company reviews the allowance for doubtful accounts on a quarterly basis. Account balances are charged off against the allowance when the Company believes it is probable the receivable will not be recovered. The following table presents changes in the allowance for doubtful accounts (in thousands): Year Ended December 31, 2018 2017 2016 Beginning balance $ 2,257 $ 2,574 $ 686 Add: bad debt expense 2,115 4,289 1,890 Less: write-offs, net of recoveries (1,399 ) (4,606 ) (2 ) Ending balance $ 2,973 $ 2,257 $ 2,574 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 following estimated useful lives: Years Computer equipment 2 – 3 Purchased software 3 – 5 Furniture, fixtures and office equipment 5 Leasehold improvements * * Leasehold improvements are amortized on a straight-line basis over the term of the lease, or the useful life of the assets, whichever is shorter. Repair and maintenance costs are charged to expense as incurred, while renewals and improvements are capitalized. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation are removed from the accounts and any resulting gain or loss is reflected in the Company’s operating results. Capitalized Software Development Costs The Company capitalizes certain costs associated with creating and enhancing internally developed software related to the Company’s technology infrastructure. These costs include personnel and related employee benefits expenses for employees who are directly associated with and who devote time to software development projects, and external direct costs of materials and services consumed in developing or obtaining the software. Software development costs that do not qualify for capitalization, as further discussed below, are expensed as incurred and recorded in technology and development expenses in the consolidated statements of operations. Software development activities typically consist of three stages: (1) the planning phase; (2) the application and infrastructure development stage; and (3) the post implementation stage. Costs incurred in the planning and post implementation phases, including costs associated with the post-configuration training and repairs and maintenance of the developed technologies, are expensed as incurred. The Company capitalizes costs associated with software developed when the preliminary project stage is completed, management implicitly or explicitly authorizes and commits to funding the project and it is probable that the project will be completed and perform as intended. Costs incurred in the application and infrastructure development phases, including significant enhancements and upgrades, are capitalized. Capitalization ends once a project is substantially complete and the software is ready for its intended purpose. Software development costs are amortized using a straight-line method over the estimated useful life of two years, commencing when the software is ready for its intended use. The straight-line recognition method approximates the manner in which the expected benefit will be derived. The Company does not transfer ownership of its internally developed software, or lease its software, to third parties. On July 1, 2018, the Company early adopted ASU No. 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract , using the prospective method. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Previously, all implementation costs for a hosting arrangement that was a service contract were expensed when incurred. Cloud computing arrangements (“CCAs”), such as software as a service and other hosting arrangements, are evaluated for capitalized implementation costs in a similar manner as capitalized software development costs. If a CCA includes a software license, the software license element of the arrangement is accounted for in a manner consistent with the acquisition of other software licenses. If a CCA does not include a software license, the service element of the arrangement is accounted for as a service contract. The Company capitalized certain implementation costs for its CCAs that are service contracts totaling $0.9 million during 2018, which are included in . The Company amortizes capitalized implementation costs in a CCA over the life of the service contract. During 2018, the amortization of these capitalized implementation costs was immaterial. Business Combinations The results of a business 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 generally 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. Transaction costs associated with business combinations are expensed as incurred and are included in general and administrative expenses in the consolidated statements of operations. The Company performs valuations of assets acquired and liabilities assumed and allocates the purchase price to its respective assets and liabilities. Determining the fair value of assets acquired and liabilities assumed may require management to use significant judgment and estimates, including the selection of valuation methodologies, estimates of future revenue, costs 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. In October 2017, the Company paid $3.0 million in cash for certain assets of a data company accounted for as a business combination. These assets primarily consisted of acquired technology and goodwill which are included in other assets, non-current. Operating Leases The Company records rent expense for operating leases, some of which have escalating rent payments, on a straight-line basis over the lease term. The Company begins recognition of rent expense on the date of initial possession, which is generally when the Company enters the leased premises and begins to make improvements in preparation for its intended use. Some of the Company’s lease arrangements provide for concessions by the landlords, including payments for leasehold improvements and rent-free periods. The Company accounts for the difference between the straight-line rent expense and rent paid as deferred rent. Refer to the discussion below in Recent Accounting Pronouncements regarding the Company’s adoption of ASU No. 2016-02, Leases Debt Issuance Costs Debt issuance costs related to the term loans have been recorded as a reduction of the carrying amount of the debt and are amortized to interest expense using the effective interest method. Debt financing costs associated with credit facilities have been deferred and recorded in other assets, non-current and are amortized to interest expense on a straight-line basis over the term of the credit facilities. Preferred Stock Warrant Liabilities Prior to the completion of the Company’s IPO in September 2016, warrants to purchase preferred stock of the Company were accounted for as liabilities at fair value because the underlying shares of convertible preferred stock were contingently redeemable, including in the case of a deemed liquidation, which could have obligated the Company to transfer assets to the preferred stockholders. The preferred stock warrants were recorded at fair value at each balance sheet date and changes in the fair value of the preferred stock warrants during each reporting period were recorded in the Company’s consolidated statements of operations until the earlier of the exercise or expiration of the warrants or the warrants’ conversion to warrants to purchase common stock, at which time any remaining liability was reclassified to additional paid-in capital. Immediately prior to the completion of the Company’s IPO in September 2016, all of the Company’s then-outstanding convertible preferred stock warrants were remeasured to fair value and automatically converted and reclassified into Class B common stock warrants and all remaining warrants were net exercised. Fair Value of Financial Instruments Fair value is defined as 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. Fair value measurements are based on a fair value hierarchy, based on three levels of inputs, of which the first two are considered observable and the last unobservable, which are the following: Level 1—Quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company 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, such as quoted market prices for similar assets and liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability. Level 3—Unobservable inputs. Observable inputs are based on market data obtained from independent sources. The carrying amounts of accounts receivable, accounts payable, accrued expenses and other current liabilities approximate fair value due to the short-term nature of these instruments. The carrying value of the line of credit and term loans approximates fair value based on borrowing rates currently available to the Company for financing with similar terms and were determined to be Level 2. The Company’s convertible preferred stock warrants were measured using unobservable inputs that required a high level of judgment to determine fair value, and were thus classified as Level 3. Certain long lived assets including capitalized software development costs are also subject to measurement at fair value on a non-recurring basis if they are deemed to be impaired as a result of an impairment review. To date, no material impairments have been recorded on those assets. Concentration of Risk Financial instruments that potentially subject the Company to concentration of credit risk consist principally of cash and cash equivalents, marketable securities and accounts receivable. The Company maintains its cash and cash equivalents with financial institutions and its cash levels exceed the Federal Deposit Insurance Corporation (FDIC) federally insured limits. Marketable securities consist of investments in U.S. government securities, U.S. government agency securities, and high credit quality corporate debt securities. For 2018, two clients accounted for 20% and 10%, respectively, of Gross Billings. For 2017, three clients accounted for 22%, 11% and 10%, respectively, of Gross Billings. For 2016, three clients accounted for 15%, 13% and 11%, respectively, of Gross Billings. As of December 31, 2018, two clients accounted for 24% and 14%, respectively, of consolidated accounts receivable. As of December 31, 2017, three clients accounted for 33%, 14% and 11%, respectively, of consolidated accounts receivable. As of December 31, 2018, no suppliers accounted for more than 10% of consolidated accounts payable. As of December 31, 2017, one supplier accounted for 12% of consolidated accounts payable. Foreign Currency Transactions and Translation The Company has entities operating in various countries. Each of these entities’ functional currency is the U.S. Dollar. Transactions in foreign currencies are translated into U.S. Dollars at the rates of exchange in effect at the date of the transaction. Net transaction losses were approximately $1.9 million, $4.0 million, and $1.2 million for the years ended December 31, 2018, 2017 and 2016, respectively, and are included in foreign currency exchange loss, net in the accompanying consolidated statements of operations. The Company enters into forward contracts to hedge foreign currency exposures related primarily to the Company’s foreign currency denominated accounts receivable. The Company does not designate the foreign exchange forward contracts as hedges for accounting purposes and changes in the fair value of the foreign exchange forward contracts are recorded in foreign currency exchange loss, net in the accompanying consolidated statements of operations. As of December 31, 2018 and 2017, the Company had open forward contracts with aggregate notional amounts of $78.8 million and $58.4 million, respectively. The fair value of the open forward contracts was not material. The Company’s forward contracts generally have terms of 30-60 days. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No.

Net Income (Loss) Per Share Att

Net Income (Loss) Per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Income (Loss) Per Share Attributable to Common Stockholders	Note 3—Net Income (Loss) Per Share Attributable to Common Stockholders The Company has two classes of common stock, Class A and Class B. Basic and diluted net income (loss) per share attributable to common stockholders for Class A and Class B common stock were the same because they were entitled to the same liquidation and dividend rights. The computations of the numerators and denominators of the basic and diluted net income (loss) per share attributable to common stockholders are as follows (in thousands, except per share amounts): Year Ended December 31, 2018 2017 2016 Numerator: Net income $ 88,140 $ 50,798 $ 20,482 Less: Premium on repurchase of convertible preferred stock — — (47,209 ) Net income (loss) attributable to common stockholders $ 88,140 $ 50,798 $ (26,727 ) Denominator: Weighted-average shares outstanding—basic 42,442 40,262 18,280 Effect of dilutive securities: Options to purchase common stock 2,845 3,415 — Employee stock purchase plan shares 251 268 — Restricted stock 255 111 — Weighted-average shares outstanding—diluted 45,793 44,056 18,280 Basic EPS $ 2.08 $ 1.26 $ (1.46 ) Diluted EPS $ 1.92 $ 1.15 $ (1.46 ) Anti-dilutive equity awards under stock-based award plans excluded from the determination of diluted EPS 472 1,246 6,069

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, Net	Note 4—Property and Equipment, Net Major classes of property and equipment were as follows (in thousands): As of December 31, 2018 2017 Computer equipment $ 11,806 $ 4,520 Purchased software 9,777 7,735 Furniture and fixtures 8,175 4,306 Construction in progress 252 102 Leasehold improvements 18,693 9,645 48,703 26,308 Less: Accumulated depreciation (15,657 ) (8,903 ) $ 33,046 $ 17,405 Depreciation expense for 2018, 2017 and 2016 was $8.1 million, $5.3 million and $2.4 million respectively. The Company has purchased software under a financing arrangement where the Company makes installment payments over a four-year period. The Company has taken possession of the software and runs the software on its own equipment. As of December 31, 2018 and 2017, the software had a cost basis of $1.8 million. Accumulated depreciation on this software as of December 31, 2018 and 2017 was $1.6 million and $1.2 million, respectively. Depreciation expense on this software was $0.4 million for 2018, 2017 and 2016. To date, there have been no impairment charges to property and equipment.

Capitalized Software Developmen

Capitalized Software Development Costs	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
Capitalized Software Development Costs	Note 5—Capitalized Software Development Costs Capitalized software development costs, included in other assets, non-current, were as follows (in thousands): As of December 31, 2018 2017 Capitalized software development costs, gross $ 11,487 $ 6,133 Less: Accumulated amortization (3,386 ) (1,734 ) Capitalized software development costs, net $ 8,101 $ 4,399 The Company capitalized $6.9 million, $3.5 million and $2.4 million of software development costs in 2018, 2017 and 2016, respectively. Amortization expense was $3.2 million, $1.9 million and $1.4 million for 2018, 2017 and 2016, respectively. Based on the Company’s capitalized software development costs ready for intended use as of December 31, 2018, estimated amortization expense of $4.2 million and $2.0 million is expected to be recognized in 2019 and 2020, respectively. Amortization has not started on $1.9 million of capitalized software development costs that are not yet ready for intended use as of December 31, 2018.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Note 6—Fair Value Measurements Cash equivalents, consisting of money market funds and time deposits, of $172.1 million and $5.0 million at December 31, 2018 and 2017, respectively, were classified as Level 1 of the fair value hierarchy and valued using quoted market prices in active markets. The Company had no other material financial instruments that were measured at fair value at December 31, 2018 and 2017. The Company’s preferred stock warrants are recorded at fair value and were determined to be Level 3 fair value items. The changes in the fair value of preferred stock warrants are summarized below (in thousands): Balance at December 31, 2015 $ 6,927 Change in value of preferred stock warrants recorded in other expense, net 9,458 Reclassification to convertible preferred stock upon net exercise of Series Seed warrant in February 2016 (3,789 ) Conversion of preferred stock warrants to common stock warrants upon the closing of the Company’s IPO on September 26, 2016 (12,596 ) Balance at December 31, 2016 $ — In connection with the IPO in September 2016, outstanding warrants exercisable for 1,382,505 shares of convertible preferred stock were automatically converted into warrants exercisable for 460,834 shares of Class B common stock and net exercised resulting in the issuance of 448,545 shares of Class B common stock based on the IPO price of $18.00 per share and taking into account the 1-for-3 reverse stock split. The aggregate fair value of these warrants upon the closing of the IPO was $12.6 million, which was reclassified from liabilities to additional paid-in capital.

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt	Note 7—Debt Revolving Credit Facility On October 26, 2018, the Company and a syndicate of banks, led by Citibank, N.A., as agent, entered into a second amended and restated loan and security agreement (the “Second A&R Credit Agreement”). Loans under the Second A&R Credit Facility bear interest through maturity at a variable rate based upon, at the Company’s option, an annual rate of either a Base Rate or a LIBOR rate, plus an applicable margin (“Base Rate Borrowings” and “LIBOR Rate Borrowings”). The Base Rate is defined as a fluctuating interest rate equal to the greatest of (1) the federal funds rate plus 0.50%, (2) Citibank, N.A.’s prime rate, and (3) one month LIBOR rate plus 2.00%. The applicable margin is between 0.25% to 1.25% for Base Rate Borrowings and between 1.25% and 2.25% for LIBOR Rate Borrowings based on the Company maintaining certain leverage ratios. The fee for undrawn amounts under the Second A&R Credit Facility ranges, based on the applicable leverage, from 0.225% to 0.400%. The Company will also be required to pay customary letter of credit fees, as necessary. As of December 31, 2018, the Company did not have an outstanding debt balance under the Second A&R Credit Facility, and availability was $143.3 million. The Second A&R Credit Facility matures and all outstanding amounts become due and payable on May 9, 2022. The Second A&R Credit Agreement contains customary conditions to borrowings, events of default and covenants, including covenants that restrict our ability to sell assets, make changes to the nature of our business, engage in mergers or acquisitions, incur, assume or permit to exist additional indebtedness and guarantees, create or permit to exist liens, pay dividends, issue equity instruments, make distributions or redeem or repurchase capital stock or make other investments, engage in transactions with affiliates and make payments in respect of subordinated debt. The Second A&R Credit Agreement also requires the Company to maintain compliance with (a) a maximum ratio of consolidated funded debt to consolidated EBITDA of 3.50 to 1.00 and (b) a minimum ratio of consolidated EBITDA to interest expense of at least 3.00 to 1.00. As of December 31, 2018, the Company was in compliance with all covenants. The Company entered into the Second A&R Credit Agreement primarily to lower its borrowing costs and to change from an asset-based structure to a cash-flow based structure. In connection with a prior debt facility, the Company was required to pay a fee of $0.8 million upon the occurrence of the IPO in September 2016. This liquidation fee was paid upon the completion of the IPO and recorded as a component of interest expense in the consolidated statements of operations for the year ended December 31, 2016.

Capitalization

Capitalization	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Capitalization	Note 8—Capitalization In September 2016, and in preparation of the IPO and the establishment of two classes of common stock, each share of the then outstanding common stock was reclassified to Class B common stock. The Company sold Class A common stock in the IPO. The Class A and Class B have the same rights and preferences including rights to dividends, except the Class B is entitled to ten votes per share and the Class A is entitled to one vote per share. Each share of Class B common stock is convertible at any time at the option of the holder into one share of Class A common stock. In addition, each share of Class B common stock will convert automatically into one share of Class A common stock upon any transfer, except for certain transfers described in the Company’s restated certificate of incorporation, including, without limitation, certain transfers for tax and estate planning purposes. During the years ended December 31, 2018 and 2017, 3.4 million and 21.8 million shares of Class B common stock were converted to Class A common stock, respectively, and during the period from September 2016 through December 2016, 4.2 million shares of Class B common stock were converted to Class A common stock. In addition, upon the earlier of (1) the date on which the outstanding shares of Class B common stock represent less than 10% of the aggregate number of the then outstanding shares of Class A common stock and Class B common stock and (2) the affirmative vote or written consent of the holders of at least 66 2 ⁄ 3 As of December 31, 2018, the Company is authorized to issue 1,095,000,000 shares of common stock, par value $0.000001 per share, and 100,000,000 shares of preferred stock, par value, $0.000001 per share. The authorized common stock consists of 1,000,000,000 shares of Class A common stock and 95,000,000 shares of Class B common stock. No shares of preferred stock are outstanding as of December 31, 2018 and 2017. The Company’s board of directors has the discretion to determine the rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences, of each series of preferred stock. Initial Public Offering On September 26, 2016, the Company completed its IPO whereby 4,666,667 shares of Class A common stock were issued and sold by the Company and 700,000 shares of Class A common stock were sold by selling stockholders, pursuant to the underwriters’ exercise of their over-allotment option, at the IPO price of $18.00 per share. The Company received net proceeds from the offering of approximately $78.1 million after deducting underwriting discounts and commissions of $5.9 million, but before deducting offering costs of $4.5 million. The Company did not receive any proceeds from the sales of shares by the selling stockholders. In connection with the Company’s IPO: (1) all shares of the Company’s outstanding Series Seed, A-1, A-2, A-3, B and C convertible preferred stock automatically converted into an aggregate 22,078,637 shares of Class B common stock on a one for one-third basis and (2) warrants exercisable for 1,382,505 shares of convertible preferred stock were automatically converted into warrants exercisable for 460,834 shares of Class B common stock and net exercised resulting in the issuance of 448,545 shares of Class B common stock based on the IPO price of $18.00 per share and taking into account the 1-for-3 reverse stock split. In addition, upon completion of the IPO, costs associated with the IPO of $4.5 million were reclassified from other assets, non-current to additional paid-in capital. Secondary Offerings In March 2017, the Company completed a secondary offering (the “March 2017 Offering”) in which a total of 7,281,789 shares of Class A common stock were sold by certain selling stockholders to the public at a price of $35.50 per share, including 949,798 shares of Class A common stock sold to the underwriters pursuant to an option to purchase additional shares granted to them. In June 2017, the Company completed another secondary offering (the “June 2017 Offering”) in which a total of 4,846,600 shares of Class A common stock were sold by certain selling stockholders to the public at a price of $52.00 per share, including 530,148 shares of Class A common stock sold to the underwriters pursuant to an option to purchase additional shares granted to them. The Company did not receive any proceeds from either the March 2017 Offering or the June 2017 Offering. The Company incurred $0.9 million and $0.6 million in offering costs related to the March 2017 Offering and the June 2017 Offering, respectively, and these costs were included within general and administrative expenses in the condensed consolidated statements of operations during the first and second quarters of 2017, respectively. Convertible Preferred Stock In February 2016, the Company issued 11,500,587 shares of Series C convertible preferred stock for $60.0 million and used $54.0 million of the proceeds to repurchase 3,897,928 and 8,485,350 shares of Series Seed preferred stock (including shares issued upon exercise of warrant described below) and Series A preferred stock (comprising shares of Series A-1, A-2 and A-3), respectively, each at 80% of the Series C offering price per share of $5.22, and 188,786 shares of common stock at a price per share of $12.51. Warrants to purchase 808,135 shares of Seed preferred stock were net exercised, resulting in the issuance of 788,755 shares of Series Seed preferred stock of which 614,052 shares were then repurchased. The repurchase price of the convertible preferred stock, including legal costs, of $51.8 million exceeded the carrying value of $4.6 million at the date of repurchase. The repurchase price in excess of the then carrying value of the preferred stock of $47.2 million was recorded as a reduction to additional paid-in capital of $1.2 million and a reduction to retained earnings of $46.0 million. For the computation of net loss per share attributable to common stockholders for the year ended December 31, 2016, the repurchase price in excess of the then carrying value of the preferred stock of $47.2 million was recorded as a reduction to net income in computing net loss attributable to common stockholders. All shares of the Company’s outstanding Series Seed, A-1, A-2, A-3, B and C convertible preferred stock automatically converted into an aggregate 22,078,637 shares of Class B common stock on a one for one-third basis upon the completion of the Company’s IPO in September 2016. Rights and Preferences of Convertible Preferred Stock Prior to the Company’s IPO, the Company’s convertible preferred stock had voting rights that allowed the holder to vote the number of common stock equal to which such shares of preferred stock could be converted; entitled the preferred stockholders to certain dividend right; entitled the holders to preference in payment upon a liquidation event, including upon a change in control, prior to the common stock holders equal to the greater of the original purchase price plus any dividends or such amount per share as would have been payable had all shares of the preferred stock been converted into common stock; and were convertible at the option of the holder at any time or automatically upon a qualified IPO.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	Note 9—Stock-Based Compensation Stock-Based Compensation Expense Stock-based compensation expense recorded in the consolidated statements of operations was as follows (in thousands): Year Ended December 31, 2018 2017 2016 Platform operations $ 4,463 $ 2,674 $ 756 Sales and marketing 11,306 6,261 1,707 Technology and development 13,855 6,661 1,513 General and administrative 12,586 5,721 1,080 Total $ 42,210 $ 21,317 $ 5,056 Stock-Based Award Plans The Company is authorized to issue stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based and cash-based awards under its 2016 Incentive Award Plan. As of December 31, 2018, 3.6 million shares remained available for grant under the Company’s 2016 Incentive Award Plan. The number of shares authorized for grant is subject to increase each year on January 1, equal to the lesser of (a) 4% of the common stock outstanding (on an as-converted basis) on the final day of the immediately preceding calendar year and (b) such smaller number of shares as determined by the board of directors. On January 1, 2019, the number of shares authorized for grant under the Company’s 2016 Incentive Award Plan was increased by 1.8 million shares in accordance with plan provisions. Stock options granted under the Company’s stock incentive plans generally vest over four years, subject to the holder’s continued service through the vesting date, and expire no later than 10 years from the date of grant. Restricted stock awards generally vest over four years, subject to the holder’s continued service through the vesting date. Stock Options The following summarizes stock option activity: Shares Under Option (in thousands) Weighted- Average Exercise Price Weighted- Average Contractual Life (years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2017 4,745 $ 17.96 Granted 1,453 70.53 Exercised (1,446 ) 6.93 Cancelled (122 ) 50.96 Outstanding as of December 31, 2018 4,630 $ 37.03 7.6 $ 371,550 Exercisable as of December 31, 2018 1,974 $ 15.94 6.2 $ 197,629 The fair value of options on the date of grant was estimated based on the Black-Scholes option pricing model. The weighted average assumptions used to value options granted to employees for the periods presented were as follows: Year Ended December 31, 2018 2017 2016 Expected term (years) 6.0 6.0 6.0 Expected volatility 48.2 % 52.6 % 58.1 % Risk-free interest rate 2.85 % 2.03 % 1.62 % Estimated dividend yield — % — % — % The weighted average grant date fair value per share of stock options granted for the years ended December 31, 2018, 2017 and 2016 and were $35.08, $22.48 and $11.61, respectively. The total intrinsic value of options exercised during the years ended December 31, 2018, 2017 and 2016 were $119.0 million, $84.8 million and $13.7 million, respectively. Stock-based compensation expense related to stock options was $17.7 million, $7.9 million and $1.7 million for the years ended December 31, 2018, 2017 and 2016, respectively. At December 31, 2018, the Company had unrecognized employee stock-based compensation relating to stock options of approximately $68.6 million, which is expected to be recognized over a weighted-average period of 2.7 years. Restricted Stock The following summarizes restricted stock activity: Shares (in thousands) Weighted- Average Grant Date Fair Value Per Share Unvested as of December 31, 2017 418 $ 39.95 Granted 239 96.21 Vested (134 ) 41.32 Forfeited (5 ) 83.10 Unvested as of December 31, 2018 518 $ 65.14 Stock-based compensation expense related to restricted stock was $7.4 million, $2.7 million and $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively. At December 31, 2018, the Company had unrecognized employee stock-based compensation relating to restricted stock of approximately $30.8 million, which is expected to be recognized over a weighted-average period of 2.7 years. On November 30, 2018, the Company entered into a transition agreement with its Chief Operating Officer (the “Transition Agreement”), which transition period is expected to end by June 30, 2019. Among other terms of the Transition Agreement, the Company granted a performance-based restricted stock award for 24,000 shares of the Company’s common stock subject to time-and performance-based vesting (the “PSUs”). The PSUs are included in the restricted stock activity for the year ended December 31, 2018. The related compensation expense is calculated using the number of shares of common stock expected to vest based on the probability and estimated level of achievement of the performance goals. Employee Stock Purchase Plan In September 2016, the Company established an ESPP with 800,000 shares of Class A common stock available for issuance. As of December 31, 2018, 0.2 million shares remained available for grant under this plan. The number of shares authorized for grant is subject to increase each year on January 1, equal to the lesser of (a) 800,000 shares, (b) 1% of the common stock outstanding (on an as-converted basis) on the final day of the immediately preceding calendar year, and (c) such smaller number of shares as determined by the Company’s board of directors. On January 1, 2019, the number of shares available for issuance under the Company’s Employee Stock Purchase Plan was increased by 0.4 million shares in accordance with plan provisions. Under the ESPP, all eligible employees were auto-enrolled upon the IPO and each eligible employee was then permitted to authorize payroll deductions of up to 100% of their compensation to purchase shares of Class A common stock, subject to applicable ESPP and statutory limits. The ESPP provides for offering periods generally up to two years, with purchases occurring and new offering periods commencing generally every six months. The first ESPP purchase (pursuant to a truncated purchase period starting on the Company’s IPO) occurred on December 29, 2016, and subsequent purchases will generally occur on May 15th and November 15th each year. At each purchase date, employees are able to purchase shares at 85% of the lower of (1) the closing market price per share of Class A common stock on the employee’s enrollment into the applicable offering period and (2) the closing market price per share of Class A common stock on the purchase date. The ESPP has an automatic reset feature, whereby the offering period resets if the fair value of the Company’s common stock on a purchase date is less than that on the original offering date. The fair value of ESPP shares was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions: Year ended December 31, 2018 2017 2016 Expected term (years) 0.8 0.9 0.8 Expected volatility 46.2 % 45.9 % 48.9 % Risk-free interest rate 2.32 % 1.22 % 0.69 % Estimated dividend yield — % — % — % The first offering period allowed for cash contributions in addition to payroll deductions, and as a result, stock-based compensation expense for this offering period was marked-to-market. On December 14, 2016, the cash contribution feature was removed and the final mark-to-market adjustment was recorded as of this date. The ESPP has a six month holding period (12 months for the first offering period) with respect to common stock purchases. Due to the holding period, the Company applies a discount to reflect the non-transferability of the shares. Stock-based compensation expense related to ESPP totaled $17.1 million, $10.7 million and $3.3 million for the years ended December 31, 2018, 2017 and 2016, respectively. At December 31, 2018, the Company had unrecognized employee stock-based compensation relating to ESPP awards of approximately $8.8 million, which is expected to be recognized over a weighted-average period of 0.9 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Note 10—Income Taxes The following are the domestic and foreign components of the Company’s income (loss) before income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Domestic $ 115,706 $ 66,148 $ 45,904 International (9,969 ) (2,523 ) (2,070 ) Income before income taxes $ 105,737 $ 63,625 $ 43,834 The following are the components of the provision for (benefit from) income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 11,683 $ 9,944 $ 18,300 State and local 9,295 3,906 5,595 Foreign 1,720 558 64 Total current provision 22,698 14,408 23,959 Deferred: Federal (918 ) (172 ) (68 ) State and local (2,615 ) (1,382 ) (539 ) Foreign (1,568 ) (27 ) — Total deferred provision (5,101 ) (1,581 ) (607 ) Total provision for income taxes $ 17,597 $ 12,827 $ 23,352 A reconciliation of the statutory tax rate to the effective tax rate for the periods presented is as follows: Year Ended December 31, 2018 2017 2016 U.S. federal statutory income tax rate 21.0 % 35.0 % 35.0 % State and local income taxes, net of federal benefit 5.0 2.6 7.5 Foreign income at other than U.S. rates 2.1 2.7 1.1 Stock-based compensation (7.6 ) (19.0 ) 3.8 Meals and entertainment 0.5 0.4 0.3 Change in value of preferred stock warrant liabilities — — 7.6 Research and development credit (3.9 ) (2.8 ) (3.1 ) Federal deferred tax asset revaluation — 0.9 — Other permanent items (0.4 ) 0.9 0.4 Change in valuation allowance — (0.5 ) 0.7 Effective income tax rate 16.7 % 20.2 % 53.3 % Set forth below are the tax effects of temporary differences that give rise to a significant portion of the deferred tax assets and deferred tax liabilities (in thousands): As of December 31, 2018 2017 Deferred tax assets (liabilities): Reserves and allowances $ 2,465 $ 1,804 Accrued expenses 4,193 1,810 Net operating losses 854 — Research and development tax credit 1,367 774 Stock-based compensation 4,726 1,722 Deferred rent 1,425 895 Prepaid expenses (559 ) (580 ) Property and equipment (4,141 ) (2,026 ) Capitalized software development costs (1,874 ) (1,116 ) Other 4 76 Total deferred tax assets, net $ 8,460 $ 3,359 As of December 31, 2018, the Company had net operating loss carryforwards in the United Kingdom (“U.K.”) of approximately $0.9 million, which can be carried carryforward indefinitely. As of December 31, 2018, the Company had state and U.K. research and development tax credits of approximately $2.4 million and $0.2 million, respectively, which can be carried forward indefinitely. As of December 31, 2018, unremitted earnings of the subsidiaries outside of the U.S. were approximately $14.1 million, on which no state taxes have been paid. The Company’s intention is to indefinitely reinvest these earnings outside the U.S. Upon distribution of those earnings in the form of a dividend or otherwise, the Company would be subject to both state income taxes and withholding taxes payable to various foreign countries. The amounts of such tax liabilities that might be payable upon repatriation of foreign earnings are not material. As of December 31, 2018 and 2017, the Company had gross unrecognized tax benefits of approximately $4.3 million and $3.1 million, respectively, which would affect the Company’s effective tax rate if recognized. The Company classifies liabilities for unrecognized tax benefits in other liabilities, non-current. The following table presents changes in gross unrecognized tax benefits (in thousands): Year Ended December 31, 2018 2017 2016 Beginning balance $ 3,101 $ 1,007 $ — Increases related to prior year tax positions — 123 402 Decreases related to prior year tax positions (270 ) — — Increases related to current year tax positions 1,499 1,971 605 Ending balance $ 4,330 $ 3,101 $ 1,007 Interest and penalties related to the Company’s unrecognized tax benefits accrued as of December 31, 2018 were not material. The Company files U.S. federal, state, and foreign tax returns. The Company is currently under examination by the Internal Revenue Service for the years ended December 31, 2014 and 2015. The Company expects to reduce its unrecognized tax benefits by $0.2 million during the next twelve months. The 2016 and 2017 U.S. federal tax returns and 2010 through 2017 state tax returns remain open to examination. The majority of the Company’s foreign subsidiaries remain subject to examination by local taxing authorities for 2014 and subsequent years.

Segment and Geographic Informat

Segment and Geographic Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment and Geographic Information	Note 11—Segment and Geographic Information The Company has determined that it operates as one operating segment. The Company’s chief operating decision maker reviews financial information on a consolidated basis, together with certain operating and performance measures principally to make decisions about how to allocate resources and measure performance. Gross Billings, based on the billing address of the clients or client affiliates, were as follows (in thousands): Year Ended December 31, 2018 2017 2016 US $ 1,937,074 $ 1,270,116 $ 868,877 International 347,939 221,626 121,684 Total $ 2,285,013 $ 1,491,742 $ 990,561 The Company’s property and equipment, net located outside the U.S. was $7.2 million and $3.6 million as of December 31, 2018 and 2017, respectively.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 12—Commitments and Contingencies The Company has various non-cancelable operating leases primarily for its corporate and international offices. As of December 31, 2018, the Company’s non-cancelable minimum lease commitments were as follows (in thousands): Year Amount (1) 2019 $ 13,419 2020 18,746 2021 29,054 2022 23,786 2023 20,902 Thereafter 119,109 $ 225,016 (1) Includes non-cancelable minimum lease payments of $183.5 million for leases executed in 2018 that the Company plans to occupy in 2019. Rent expense for non-cancelable operating leases was $10.9 million, $8.2 million and $4.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, the Company has non-cancelable commitments to its hosting services providers, marketing contracts and commitments to providers of software as a service. Year Amount 2019 $ 33,352 2020 516 2021 230 2022 230 $ 34,328 In January 2019, the Company entered into a hosting services arrangement that includes a $200.0 million commitment through April 2024. Guarantees and Indemnification In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to clients, vendors, lessors, business partners, and other parties with respect to certain matters, including, but not limited to, losses arising out of breach of such agreements, services to be provided by the Company or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with directors and certain officers and employees that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors, officers or employees. No demands have been made upon the Company to provide indemnification under such agreements, and thus, there are no claims that the Company is aware of that could have a material effect on the Company’s balance sheet, statement of operations or statement of cash flows. Accordingly, no amounts for any obligation have been recorded as of December 31, 2018 and 2017. Litigation From time to time, the Company is subject to various legal proceedings and claims, either asserted or unasserted, that arise in the ordinary course of business. Although the outcome of the various legal proceedings and claims cannot be predicted with certainty, management does not believe that any of these proceedings or other claims will have a material adverse effect on the Company’s business, financial condition, results of operations or cash flows. Employment Contracts The Company has entered into agreements with severance terms with certain employees and officers, all of whom are employed on an at-will basis, subject to certain severance obligations in the event of certain involuntary terminations. The Company may be required to accelerate the vesting of certain stock options in the event of changes in control, as defined and involuntary terminations.

Basis of Presentation and Sum_2

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the operations of the Company and its wholly owned subsidiaries. All intercompany transactions have been eliminated in consolidation. The Company has no components of other comprehensive income (loss), and accordingly, the Company’s comprehensive income is the same as its net income for all periods presented.
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 consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ materially from these estimates. On an on-going basis, management evaluates its estimates, primarily those related to: (1) revenue recognition criteria, including the determination of revenue reporting as net versus gross in the Company’s revenue arrangements, (2) allowances for doubtful accounts, (3) the useful lives of property and equipment and capitalized software development costs, (4) income taxes, (5) assumptions used in the Black-Scholes option pricing model to determine the fair value of stock-based compensation, (6) the recognition and disclosure of contingent liabilities and (7) the assumptions used in valuing acquired assets and assumed liabilities in business combinations
Revenue Recognition	Revenue Recognition On January 1, 2018, the Company adopted Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) The Company determines revenue recognition through the following steps: • Identification of a contract with a customer; • Identification of the performance obligations in the contract; • Determination of the transaction price; • Allocation of the transaction price to the performance obligations in the contract; and • Recognition of revenue when or as the performance obligations are satisfied. The Company maintains agreements with each client and supplier in the form of master service agreements, which set out the terms of the relationship and access to the Company’s platform. The Company’s performance obligation is to provide the use of its platform to clients to develop ad campaigns and select the advertising inventory, data and other add-on features. The Company charges clients a platform fee, based on a percentage of a client’s purchases through the platform, and the transaction price is determined based on the consideration to which it expects to be entitled in exchange for the completion of a transaction, that is, when a bid is won. The Company reports revenue net of amounts it pays suppliers for the cost of advertising inventory, third-party data and other add-on features (collectively, “Supplier Features”). The determination of whether the Company is the principal or agent, and hence whether to report revenue on a gross basis for the amount of the Supplier Features the clients purchase using the platform plus the Company’s platform fees or on a net basis for the amount of platform fees charged to the client, requires judgment. The Company determined that it is not primarily responsible for the purchase of Supplier Features, but rather, it is primarily responsible to provide a platform that enables clients to bid on advertising inventory, and use data and other add-on features in designing and executing their campaigns. The Company does not control the Supplier Features prior to the purchase by the client, and it does not have pricing latitude with respect to the cost of such features. The platform fee the Company charges clients is a percentage of their purchases through its platform, similar to a commission, and the platform fee is not contingent on the results of an advertising campaign. Based on these and other factors, the Company determined that it is not the principal in the purchase and sale of Supplier Features in all of its arrangements, and therefore, it reports revenue on a net basis for the platform fees charged to clients. The Company generally bills clients for the gross amount of Supplier Features they purchase through its platform and the platform fees, net of allowances (“Gross Billings”). Some of the Company’s clients have payment relationships directly with advertising inventory suppliers in which case the Company only bills these clients for third-party data, other add-on features and its platform fees. The Company invoices its clients on a monthly basis for the purchases occurring during the month. Invoice payment terms, negotiated on a client-by-client basis, are typically between 30 to 90 days. However, for certain agency clients with sequential liability terms, payments are not due to the Company until such agency client has received payment from its customers who are advertisers. The Company’s accounts receivable are recorded at the amount of Gross Billings for the amounts it is responsible to collect, and accounts payable are recorded at the net amount payable to suppliers. Accordingly, both accounts receivable and accounts payable appear large in relation to revenue reported on a net basis. Refer to Note 11 for geographic information related to Gross Billings.
Operating Expenses	Operating Expenses The Company classifies its operating expenses into four categories: Platform Operations. Platform operations expense consists of expenses related to hosting the Company’s platform, which includes “internet traffic” associated with the viewing of available impressions or queries per second (QPS) and providing support to clients. Platform operations expense includes hosting costs, personnel costs, and amortization of acquired technology and capitalized software costs for the development of the Company’s platform, including allocated overhead. Personnel costs included in platform operations include salaries, bonuses, stock-based compensation, and employee benefit costs, and are primarily attributable to personnel who provide the Company’s clients with support using the Company’s platform and the personnel who support the Company’s platform. The Company capitalizes certain costs associated with the development of the Company’s platform and amortizes these costs over their estimated useful lives in platform operations expense. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Sales and Marketing. Sales and marketing expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation, employee benefits costs and commission costs, for the Company’s sales and marketing personnel. Sales and marketing expense also includes costs for market development programs, advertising, promotional and other marketing activities, and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Commissions costs are expensed as incurred. Technology and Development. The Company’s technology and development expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation and employee benefits costs, third-party consultant costs associated with the ongoing development and maintenance of the Company’s platform and integrations with our advertising and data inventory suppliers, amortization of capitalized third-party software used in the development of the Company’s platform and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount. Technology and development costs are expensed as incurred, except to the extent that such costs are associated with software development that qualifies for capitalization, which are then recorded as capitalized software development costs included in other assets, non-current on the Company’s consolidated balance sheet. The Company amortizes capitalized software development costs relating to the Company’s platform to platform operations expense. General and Administrative. The Company’s general and administrative expense consists primarily of personnel costs, including salaries, bonuses, stock-based compensation, and employee benefits costs associated with the Company’s executive, finance, legal, human resources, compliance, and other administrative personnel, as well as accounting and legal professional services fees, bad debt expense and allocated overhead. The Company allocates overhead such as information technology infrastructure, rent and occupancy charges based on headcount.
Stock-Based Compensation	Stock-Based Compensation Compensation expense related to stock options, restricted stock awards and units, which are referred to collectively as restricted stock, and awards granted under the Company’s employee stock purchase plan (“ESPP”), is measured and recognized in the consolidated financial statements based on the fair value of the awards granted. The fair value of each option award is estimated on the grant date using the Black-Scholes option-pricing model. The fair value of restricted stock is calculated using the closing market price of the Company’s common stock on the date of grant, and for performance-based restricted stock, we also assess the probability of targets being achieved in determining the amount of expense to recognize. Stock-based compensation expense related to stock options and restricted stock is recognized on a straight-line basis over the requisite service periods of the awards, which is generally four years. For performance-based restricted stock, expense is recognized on a graded-vesting attribution basis over the requisite service period of the award and is adjusted in subsequent reporting periods if the assessed probability or estimated level of achievement of the performance goals changes. Stock-based compensation expense for ESPP awards is recognized on a graded-vesting attribution basis over the requisite service period of each award. Determining the fair value of stock options and ESPP awards requires judgment. The Company’s use of the Black-Scholes option pricing model requires the input of subjective assumptions, including the fair value of the underlying common stock for periods prior to the completion of the Company’s IPO in September 2016, the expected term of the option, the expected volatility of the Company’s common stock, risk-free interest rates, and the expected dividend yield of the Company’s common stock. The assumptions used in the Company’s option-pricing model represent management’s best estimates. These estimates involve inherent uncertainties and the application of management’s judgment. These assumptions and estimates are as follows: Fair Value of Common Stock. For stock options granted subsequent to the Company’s IPO in September 2016 and ESPP awards, the fair value of common stock is based on the closing price of its common stock as reported on the NASDAQ Global Market on the grant date. Prior to the IPO in September 2016, the board of directors determined the fair value of the common stock at the time of the grant of options by considering a number of objective and subjective factors including the Company’s actual operating and financial performance, market conditions and performance of comparable publicly traded companies, developments and milestones in the Company, the likelihood of achieving a liquidity event and transactions involving the Company’s preferred or common stock, among other factors. The fair value was determined in accordance with applicable elements of the practice aid issued by the American Institute of Certified Public Accountants, . Risk The risk-free interest rate is based on the yields of U.S. Treasury securities with maturities approximating the expected term of the awards. Expected Term. Given insufficient historical data relating to stock option exercises, to determine the expected term, the Company applies the 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. For ESPP awards, the expected term is the time period from the grant date to the respective purchase dates included within each offering period. Volatility. Because the Company has a short trading history for its common stock, the Company determines the price volatility based on a blend of the historical volatilities of a publicly traded peer group, implied volatilities from its traded options, and its historical volatility, based on daily price observations over a period equivalent to the expected term of the award. Dividend Yield. The dividend yield assumption is based on the Company’s history and current expectations of dividend payouts. The Company has never declared or paid any cash dividends on its common stock and does not anticipate paying any cash dividends in the foreseeable future, so the Company used an expected dividend yield of zero. During the year ended December 31, 2016, the Company early adopted Accounting Standards Update (“ASU”) No. 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share—Based Payment Accounting The Company will continue to use judgment in evaluating the assumptions related to the Company’s stock-based compensation. Future expense amounts for any particular period could be affected by changes in the Company’s assumptions or market conditions.
Income Taxes	Income Taxes Deferred income tax assets and liabilities are determined based upon the net tax effects of the differences between the Company’s consolidated financial statements carrying amounts and the tax basis of assets and liabilities and are measured using the enacted tax rate expected to apply to taxable income in the years in which the differences are expected to be reversed. A valuation allowance is used to reduce some or all of the deferred tax assets if, based upon the weight of available evidence, it is more likely than not that those deferred tax assets will not be realized. The Company recognizes the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized. The Company recognizes interest and penalties accrued related to its uncertain tax positions in its income tax provision in the accompanying consolidated statements of operations. In January 2018, the FASB released guidance on the accounting for tax on the global intangible low-taxed income ("GILTI") provisions of the Tax Cuts and Jobs Act (the "Act"). The GILTI provisions impose a tax on foreign income in excess of a deemed return on tangible assets of foreign corporations. The guidance indicates that either accounting for deferred taxes related to GILTI inclusions or to treat any taxes on GILTI inclusions as period cost are both acceptable methods subject to an accounting policy election. Effective the first quarter of 2018, the Company elected to treat any potential GILTI inclusions as a period cost.
Net Income (Loss) Per Share Attributable to Common Stockholders	Net Income (Loss) Per Share Attributable to Common Stockholders Basic net income (loss) per share attributable to common stockholders is calculated by dividing the net income (loss) attributable to common stockholders by the weighted-average number of shares of common stock outstanding. The Company applies the two-class method to allocate net income (loss) between common and other participating securities based on their participation rights. Prior to the conversion of preferred stock to common stock concurrent with the IPO in September 2016 and because the holders of the Company’s convertible preferred stock were entitled to participate in dividends, the Company allocated net income to common and preferred stock based on their respective rights to receive dividends, whether or not declared. For 2016, the excess of the repurchase price of preferred stock over its carrying value has been recorded as a reduction to net income to determine net loss attributable to common stockholders. Diluted net income (loss) per share attributable to common stockholders adjusts the basic net income (loss) per share attributable to common stockholders and the weighted-average number of shares of common stock outstanding for the potentially dilutive impact of stock options, restricted stock, ESPP and warrants, using the treasury-stock method, and convertible preferred stock using the as-if-converted method.
Cash, Cash Equivalents and Marketable Securities	Cash, Cash Equivalents and Marketable Securities The Company considers all short-term highly liquid investments with an original maturity of three months or less to be cash equivalents. Cash equivalents, consisting of money market funds and time deposits, are carried at fair value. Refer to Note 6—Fair Value Measurements for additional information regarding the fair value of cash equivalents. Subsequent to December 31, 2018, the Company used existing cash and cash equivalents to purchase $50.9 million in marketable securities classified as short-term investments. The Company classifies its marketable securities as available-for-sale investments in its current assets because they represent investments of cash available for current operations. Available-for-sale investments are carried at fair value with any unrealized gains and losses, net of taxes, included in accumulated other comprehensive income (loss) in stockholders' equity. Unrealized losses are charged against other expense when a decline in fair value is determined to be other-than-temporary. The Company determines realized gains or losses on sale of marketable securities on a specific identification method, and records such gains or losses as other expense (income).
Accounts Receivable and Allowance for Doubtful Accounts	Accounts Receivable and Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount, are unsecured and do not bear interest. The Company performs ongoing credit evaluations of its clients and certain advertisers when the Company’s agreements with its clients contain sequential liability terms that provide that the client payments are not due to the Company until the client has received payment from its customers who are advertisers. The allowance for doubtful accounts is based on the best estimate of the amount of probable credit losses in existing accounts receivable. The allowance for doubtful accounts is determined based on historical collection experience and the review in each period of the status of the then-outstanding accounts receivable, while taking into consideration current client information, subsequent collection history and other relevant data. The Company reviews the allowance for doubtful accounts on a quarterly basis. Account balances are charged off against the allowance when the Company believes it is probable the receivable will not be recovered. The following table presents changes in the allowance for doubtful accounts (in thousands): Year Ended December 31, 2018 2017 2016 Beginning balance $ 2,257 $ 2,574 $ 686 Add: bad debt expense 2,115 4,289 1,890 Less: write-offs, net of recoveries (1,399 ) (4,606 ) (2 ) Ending balance $ 2,973 $ 2,257 $ 2,574
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 following estimated useful lives: Years Computer equipment 2 – 3 Purchased software 3 – 5 Furniture, fixtures and office equipment 5 Leasehold improvements * * Leasehold improvements are amortized on a straight-line basis over the term of the lease, or the useful life of the assets, whichever is shorter. Repair and maintenance costs are charged to expense as incurred, while renewals and improvements are capitalized. When assets are retired or otherwise disposed of, the cost and related accumulated depreciation are removed from the accounts and any resulting gain or loss is reflected in the Company’s operating results.
Capitalized Software Development Costs	Capitalized Software Development Costs The Company capitalizes certain costs associated with creating and enhancing internally developed software related to the Company’s technology infrastructure. These costs include personnel and related employee benefits expenses for employees who are directly associated with and who devote time to software development projects, and external direct costs of materials and services consumed in developing or obtaining the software. Software development costs that do not qualify for capitalization, as further discussed below, are expensed as incurred and recorded in technology and development expenses in the consolidated statements of operations. Software development activities typically consist of three stages: (1) the planning phase; (2) the application and infrastructure development stage; and (3) the post implementation stage. Costs incurred in the planning and post implementation phases, including costs associated with the post-configuration training and repairs and maintenance of the developed technologies, are expensed as incurred. The Company capitalizes costs associated with software developed when the preliminary project stage is completed, management implicitly or explicitly authorizes and commits to funding the project and it is probable that the project will be completed and perform as intended. Costs incurred in the application and infrastructure development phases, including significant enhancements and upgrades, are capitalized. Capitalization ends once a project is substantially complete and the software is ready for its intended purpose. Software development costs are amortized using a straight-line method over the estimated useful life of two years, commencing when the software is ready for its intended use. The straight-line recognition method approximates the manner in which the expected benefit will be derived. The Company does not transfer ownership of its internally developed software, or lease its software, to third parties. On July 1, 2018, the Company early adopted ASU No. 2018-15, Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract , using the prospective method. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Previously, all implementation costs for a hosting arrangement that was a service contract were expensed when incurred. Cloud computing arrangements (“CCAs”), such as software as a service and other hosting arrangements, are evaluated for capitalized implementation costs in a similar manner as capitalized software development costs. If a CCA includes a software license, the software license element of the arrangement is accounted for in a manner consistent with the acquisition of other software licenses. If a CCA does not include a software license, the service element of the arrangement is accounted for as a service contract. The Company capitalized certain implementation costs for its CCAs that are service contracts totaling $0.9 million during 2018, which are included in . The Company amortizes capitalized implementation costs in a CCA over the life of the service contract. During 2018, the amortization of these capitalized implementation costs was immaterial.
Business Combinations	Business Combinations The results of a business 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 generally 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. Transaction costs associated with business combinations are expensed as incurred and are included in general and administrative expenses in the consolidated statements of operations. The Company performs valuations of assets acquired and liabilities assumed and allocates the purchase price to its respective assets and liabilities. Determining the fair value of assets acquired and liabilities assumed may require management to use significant judgment and estimates, including the selection of valuation methodologies, estimates of future revenue, costs 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. In October 2017, the Company paid $3.0 million in cash for certain assets of a data company accounted for as a business combination. These assets primarily consisted of acquired technology and goodwill which are included in other assets, non-current.
Operating Leases	Operating Leases The Company records rent expense for operating leases, some of which have escalating rent payments, on a straight-line basis over the lease term. The Company begins recognition of rent expense on the date of initial possession, which is generally when the Company enters the leased premises and begins to make improvements in preparation for its intended use. Some of the Company’s lease arrangements provide for concessions by the landlords, including payments for leasehold improvements and rent-free periods. The Company accounts for the difference between the straight-line rent expense and rent paid as deferred rent. Refer to the discussion below in Recent Accounting Pronouncements regarding the Company’s adoption of ASU No. 2016-02, Leases
Debt Issuance Costs	Debt Issuance Costs Debt issuance costs related to the term loans have been recorded as a reduction of the carrying amount of the debt and are amortized to interest expense using the effective interest method. Debt financing costs associated with credit facilities have been deferred and recorded in other assets, non-current and are amortized to interest expense on a straight-line basis over the term of the credit facilities.
Preferred Stock Warrant Liabilities	Preferred Stock Warrant Liabilities Prior to the completion of the Company’s IPO in September 2016, warrants to purchase preferred stock of the Company were accounted for as liabilities at fair value because the underlying shares of convertible preferred stock were contingently redeemable, including in the case of a deemed liquidation, which could have obligated the Company to transfer assets to the preferred stockholders. The preferred stock warrants were recorded at fair value at each balance sheet date and changes in the fair value of the preferred stock warrants during each reporting period were recorded in the Company’s consolidated statements of operations until the earlier of the exercise or expiration of the warrants or the warrants’ conversion to warrants to purchase common stock, at which time any remaining liability was reclassified to additional paid-in capital. Immediately prior to the completion of the Company’s IPO in September 2016, all of the Company’s then-outstanding convertible preferred stock warrants were remeasured to fair value and automatically converted and reclassified into Class B common stock warrants and all remaining warrants were net exercised.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined as 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. Fair value measurements are based on a fair value hierarchy, based on three levels of inputs, of which the first two are considered observable and the last unobservable, which are the following: Level 1—Quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company 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, such as quoted market prices for similar assets and liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the asset or liability. Level 3—Unobservable inputs. Observable inputs are based on market data obtained from independent sources. The carrying amounts of accounts receivable, accounts payable, accrued expenses and other current liabilities approximate fair value due to the short-term nature of these instruments. The carrying value of the line of credit and term loans approximates fair value based on borrowing rates currently available to the Company for financing with similar terms and were determined to be Level 2. The Company’s convertible preferred stock warrants were measured using unobservable inputs that required a high level of judgment to determine fair value, and were thus classified as Level 3. Certain long lived assets including capitalized software development costs are also subject to measurement at fair value on a non-recurring basis if they are deemed to be impaired as a result of an impairment review. To date, no material impairments have been recorded on those assets.
Concentration of Risk	Concentration of Risk Financial instruments that potentially subject the Company to concentration of credit risk consist principally of cash and cash equivalents, marketable securities and accounts receivable. The Company maintains its cash and cash equivalents with financial institutions and its cash levels exceed the Federal Deposit Insurance Corporation (FDIC) federally insured limits. Marketable securities consist of investments in U.S. government securities, U.S. government agency securities, and high credit quality corporate debt securities. For 2018, two clients accounted for 20% and 10%, respectively, of Gross Billings. For 2017, three clients accounted for 22%, 11% and 10%, respectively, of Gross Billings. For 2016, three clients accounted for 15%, 13% and 11%, respectively, of Gross Billings. As of December 31, 2018, two clients accounted for 24% and 14%, respectively, of consolidated accounts receivable. As of December 31, 2017, three clients accounted for 33%, 14% and 11%, respectively, of consolidated accounts receivable. As of December 31, 2018, no suppliers accounted for more than 10% of consolidated accounts payable. As of December 31, 2017, one supplier accounted for 12% of consolidated accounts payable.
Foreign Currency Transactions and Translation	Foreign Currency Transactions and Translation The Company has entities operating in various countries. Each of these entities’ functional currency is the U.S. Dollar. Transactions in foreign currencies are translated into U.S. Dollars at the rates of exchange in effect at the date of the transaction. Net transaction losses were approximately $1.9 million, $4.0 million, and $1.2 million for the years ended December 31, 2018, 2017 and 2016, respectively, and are included in foreign currency exchange loss, net in the accompanying consolidated statements of operations. The Company enters into forward contracts to hedge foreign currency exposures related primarily to the Company’s foreign currency denominated accounts receivable. The Company does not designate the foreign exchange forward contracts as hedges for accounting purposes and changes in the fair value of the foreign exchange forward contracts are recorded in foreign currency exchange loss, net in the accompanying consolidated statements of operations. As of December 31, 2018 and 2017, the Company had open forward contracts with aggregate notional amounts of $78.8 million and $58.4 million, respectively. The fair value of the open forward contracts was not material. The Company’s forward contracts generally have terms of 30-60 days.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases The original guidance required application on a modified retrospective basis to the earliest period presented. In August 2018, the FASB issued Leases (Topic 842): Targeted Improvements , which includes an option to not restate comparative periods in transition but rather to elect to use the effective date of , as the date of initial application of transition. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Compensation-Stock Compensation (Topic 718), Improvements to Nonemployee Share-based Payments This guidance is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period. Early adoption is permitted. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement-Disclosure Framework (Topic 820).

Basis of Presentation and Sum_3

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Changes in Allowance for Doubtful Accounts	The following table presents changes in the allowance for doubtful accounts (in thousands): Year Ended December 31, 2018 2017 2016 Beginning balance $ 2,257 $ 2,574 $ 686 Add: bad debt expense 2,115 4,289 1,890 Less: write-offs, net of recoveries (1,399 ) (4,606 ) (2 ) Ending balance $ 2,973 $ 2,257 $ 2,574
Schedule of Useful lives of PPE	Depreciation is computed using the straight-line method based upon the following estimated useful lives: Years Computer equipment 2 – 3 Purchased software 3 – 5 Furniture, fixtures and office equipment 5 Leasehold improvements * * Leasehold improvements are amortized on a straight-line basis over the term of the lease, or the useful life of the assets, whichever is shorter.

Net Income (Loss) Per Share A_2

Net Income (Loss) Per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Computation of numerators and denominators of basic and diluted net income (loss) per share attributable to common stockholders	The computations of the numerators and denominators of the basic and diluted net income (loss) per share attributable to common stockholders are as follows (in thousands, except per share amounts): Year Ended December 31, 2018 2017 2016 Numerator: Net income $ 88,140 $ 50,798 $ 20,482 Less: Premium on repurchase of convertible preferred stock — — (47,209 ) Net income (loss) attributable to common stockholders $ 88,140 $ 50,798 $ (26,727 ) Denominator: Weighted-average shares outstanding—basic 42,442 40,262 18,280 Effect of dilutive securities: Options to purchase common stock 2,845 3,415 — Employee stock purchase plan shares 251 268 — Restricted stock 255 111 — Weighted-average shares outstanding—diluted 45,793 44,056 18,280 Basic EPS $ 2.08 $ 1.26 $ (1.46 ) Diluted EPS $ 1.92 $ 1.15 $ (1.46 ) Anti-dilutive equity awards under stock-based award plans excluded from the determination of diluted EPS 472 1,246 6,069

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Schedule of major classes of property and equipment	Major classes of property and equipment were as follows (in thousands): As of December 31, 2018 2017 Computer equipment $ 11,806 $ 4,520 Purchased software 9,777 7,735 Furniture and fixtures 8,175 4,306 Construction in progress 252 102 Leasehold improvements 18,693 9,645 48,703 26,308 Less: Accumulated depreciation (15,657 ) (8,903 ) $ 33,046 $ 17,405

Capitalized Software Developm_2

Capitalized Software Development Costs (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
Summary of Capitalized Software Development Costs	Capitalized software development costs, included in other assets, non-current, were as follows (in thousands): As of December 31, 2018 2017 Capitalized software development costs, gross $ 11,487 $ 6,133 Less: Accumulated amortization (3,386 ) (1,734 ) Capitalized software development costs, net $ 8,101 $ 4,399

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Summary of changes in the fair value of preferred stock warrants	The changes in the fair value of preferred stock warrants are summarized below (in thousands): Balance at December 31, 2015 $ 6,927 Change in value of preferred stock warrants recorded in other expense, net 9,458 Reclassification to convertible preferred stock upon net exercise of Series Seed warrant in February 2016 (3,789 ) Conversion of preferred stock warrants to common stock warrants upon the closing of the Company’s IPO on September 26, 2016 (12,596 ) Balance at December 31, 2016 $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense by Operating Expense Category	Stock-Based Compensation Expense Stock-based compensation expense recorded in the consolidated statements of operations was as follows (in thousands): Year Ended December 31, 2018 2017 2016 Platform operations $ 4,463 $ 2,674 $ 756 Sales and marketing 11,306 6,261 1,707 Technology and development 13,855 6,661 1,513 General and administrative 12,586 5,721 1,080 Total $ 42,210 $ 21,317 $ 5,056
Summary of Stock Option Activity	The following summarizes stock option activity: Shares Under Option (in thousands) Weighted- Average Exercise Price Weighted- Average Contractual Life (years) Aggregate Intrinsic Value (in thousands) Outstanding as of December 31, 2017 4,745 $ 17.96 Granted 1,453 70.53 Exercised (1,446 ) 6.93 Cancelled (122 ) 50.96 Outstanding as of December 31, 2018 4,630 $ 37.03 7.6 $ 371,550 Exercisable as of December 31, 2018 1,974 $ 15.94 6.2 $ 197,629
Schedule of Weighted-Average Assumptions Used to Value Options Granted to Employees	The fair value of options on the date of grant was estimated based on the Black-Scholes option pricing model. The weighted average assumptions used to value options granted to employees for the periods presented were as follows: Year Ended December 31, 2018 2017 2016 Expected term (years) 6.0 6.0 6.0 Expected volatility 48.2 % 52.6 % 58.1 % Risk-free interest rate 2.85 % 2.03 % 1.62 % Estimated dividend yield — % — % — %
Summary of Nonvested Restricted Stock Activity	The following summarizes restricted stock activity: Shares (in thousands) Weighted- Average Grant Date Fair Value Per Share Unvested as of December 31, 2017 418 $ 39.95 Granted 239 96.21 Vested (134 ) 41.32 Forfeited (5 ) 83.10 Unvested as of December 31, 2018 518 $ 65.14
Schedule of Weighted-Average Assumptions Used to Estimate the Fair Value of ESPP Shares	The fair value of ESPP shares was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions: Year ended December 31, 2018 2017 2016 Expected term (years) 0.8 0.9 0.8 Expected volatility 46.2 % 45.9 % 48.9 % Risk-free interest rate 2.32 % 1.22 % 0.69 % Estimated dividend yield — % — % — %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Domestic and Foreign Components of Income (Loss) Before Income Taxes	The following are the domestic and foreign components of the Company’s income (loss) before income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Domestic $ 115,706 $ 66,148 $ 45,904 International (9,969 ) (2,523 ) (2,070 ) Income before income taxes $ 105,737 $ 63,625 $ 43,834
Components of Provision for (Benefit from) Income Taxes	The following are the components of the provision for (benefit from) income taxes (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 11,683 $ 9,944 $ 18,300 State and local 9,295 3,906 5,595 Foreign 1,720 558 64 Total current provision 22,698 14,408 23,959 Deferred: Federal (918 ) (172 ) (68 ) State and local (2,615 ) (1,382 ) (539 ) Foreign (1,568 ) (27 ) — Total deferred provision (5,101 ) (1,581 ) (607 ) Total provision for income taxes $ 17,597 $ 12,827 $ 23,352
Reconciliation of Statutory Tax Rate to Effective Tax Rate	A reconciliation of the statutory tax rate to the effective tax rate for the periods presented is as follows: Year Ended December 31, 2018 2017 2016 U.S. federal statutory income tax rate 21.0 % 35.0 % 35.0 % State and local income taxes, net of federal benefit 5.0 2.6 7.5 Foreign income at other than U.S. rates 2.1 2.7 1.1 Stock-based compensation (7.6 ) (19.0 ) 3.8 Meals and entertainment 0.5 0.4 0.3 Change in value of preferred stock warrant liabilities — — 7.6 Research and development credit (3.9 ) (2.8 ) (3.1 ) Federal deferred tax asset revaluation — 0.9 — Other permanent items (0.4 ) 0.9 0.4 Change in valuation allowance — (0.5 ) 0.7 Effective income tax rate 16.7 % 20.2 % 53.3 %
Tax Effects of Temporary Differences that Give Rise to a Significant Portion of Deferred Tax Assets and Deferred Tax Liabilities	Set forth below are the tax effects of temporary differences that give rise to a significant portion of the deferred tax assets and deferred tax liabilities (in thousands): As of December 31, 2018 2017 Deferred tax assets (liabilities): Reserves and allowances $ 2,465 $ 1,804 Accrued expenses 4,193 1,810 Net operating losses 854 — Research and development tax credit 1,367 774 Stock-based compensation 4,726 1,722 Deferred rent 1,425 895 Prepaid expenses (559 ) (580 ) Property and equipment (4,141 ) (2,026 ) Capitalized software development costs (1,874 ) (1,116 ) Other 4 76 Total deferred tax assets, net $ 8,460 $ 3,359
Schedule of Changes in Gross Unrecognized Tax Benefits	The following table presents changes in gross unrecognized tax benefits (in thousands): Year Ended December 31, 2018 2017 2016 Beginning balance $ 3,101 $ 1,007 $ — Increases related to prior year tax positions — 123 402 Decreases related to prior year tax positions (270 ) — — Increases related to current year tax positions 1,499 1,971 605 Ending balance $ 4,330 $ 3,101 $ 1,007

Segment and Geographic Inform_2

Segment and Geographic Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Gross Billings, Based on Billing Address of Clients or Client Affiliates	Gross Billings, based on the billing address of the clients or client affiliates, were as follows (in thousands): Year Ended December 31, 2018 2017 2016 US $ 1,937,074 $ 1,270,116 $ 868,877 International 347,939 221,626 121,684 Total $ 2,285,013 $ 1,491,742 $ 990,561

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Minimum Lease Commitments under Non-cancelable Operating Leases	As of December 31, 2018, the Company’s non-cancelable minimum lease commitments were as follows (in thousands): Year Amount (1) 2019 $ 13,419 2020 18,746 2021 29,054 2022 23,786 2023 20,902 Thereafter 119,109 $ 225,016 (1) Includes non-cancelable minimum lease payments of $183.5 million for leases executed in 2018 that the Company plans to occupy in 2019.
Schedule of Purchase Obligations	Year Amount 2019 $ 33,352 2020 516 2021 230 2022 230 $ 34,328

Basis of Presentation and Sum_4

Basis of Presentation and Summary of Significant Accounting Policies - Revenue Recognition (Detail)	12 Months Ended
	Dec. 31, 2018
Accounting Standards Update 2014-09
Disaggregation Of Revenue [Line Items]
Revenue, performance obligation, description of payment terms	The Company invoices its clients on a monthly basis for the purchases occurring during the month. Invoice payment terms, negotiated on a client-by-client basis, are typically between 30 to 90 days. However, for certain agency clients with sequential liability terms, payments are not due to the Company until such agency client has received payment from its customers who are advertisers.

Basis of Presentation and Sum_5

Basis of Presentation and Summary of Significant Accounting Policies - Stock-Based Compensation (Detail)	12 Months Ended
	Dec. 31, 2018
Basis of Presentation and Summary of Significant Accounting Policies
Requisite service period	4 years
Dividend yield	0.00%

Basis of Presentation and Sum_6

Basis of Presentation and Summary of Significant Accounting Policies - Cash, Cash Equivalents and Marketable Securities (Detail) $ in Millions	Feb. 21, 2019USD ($)
Subsequent Event
Marketable Securities [Line Items]
Investments in marketable securities	$ 50.9

Basis of Presentation and Sum_7

Basis of Presentation and Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts receivable [Rollforward]
Beginning balance	$ 2,257	$ 2,574	$ 686
Add: bad debt expense	2,115	4,289	1,890
Less: write-offs, net of recoveries	(1,399)	(4,606)	(2)
Ending balance	$ 2,973	$ 2,257	$ 2,574

Basis of Presentation and Sum_8

Basis of Presentation and Summary of Significant Accounting Policies - Property and Equipment, Net (Detail)	12 Months Ended
	Dec. 31, 2018
Computer Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Computer Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Purchased Software | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Purchased Software | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Furniture, fixtures and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		[1]
[1]	Leasehold improvements are amortized on a straight-line basis over the term of the lease, or the useful life of the assets, whichever is shorter.

Basis of Presentation and Sum_9

Basis of Presentation and Summary of Significant Accounting Policies - Capitalized Software Development Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Basis of Presentation and Summary of Significant Accounting Policies
Software development cost, amortization period	2 years
Capitalized Software Development
Basis of Presentation and Summary of Significant Accounting Policies
Service contracts included in other assets , noncurrent	$ 8,101	$ 4,399
ASU 2018-15 | Capitalized Software Development
Basis of Presentation and Summary of Significant Accounting Policies
Service contracts included in other assets , noncurrent	$ 900

Basis of Presentation and Su_10

Basis of Presentation and Summary of Significant Accounting Policies - Business Combinations (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017
Basis of Presentation and Summary of Significant Accounting Policies
Cash paid for certain assets acquisition	$ 3,000	$ 3,000

Basis of Presentation and Su_11

Basis of Presentation and Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)
Basis of Presentation and Summary of Significant Accounting Policies
Impairments of long-lived assets	$ 0

Basis of Presentation and Su_12

Basis of Presentation and Summary of Significant Accounting Policies - Concentration of Risk (Detail)	12 Months Ended
	Dec. 31, 2018ClientSupplier	Dec. 31, 2017ClientSupplier	Dec. 31, 2016Client
Gross Billings
Concentration Risk [Line Items]
Number of client	2	3	3
Gross Billings | Customer One
Concentration Risk [Line Items]
Concentration risk, percentage	20.00%	22.00%	15.00%
Gross Billings | Customer Two
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	11.00%	13.00%
Gross Billings | Customer Three
Concentration Risk [Line Items]
Concentration risk, percentage		10.00%	11.00%
Consolidated Accounts Receivable
Concentration Risk [Line Items]
Number of client	2	3
Consolidated Accounts Receivable | Customer One
Concentration Risk [Line Items]
Concentration risk, percentage	24.00%	33.00%
Consolidated Accounts Receivable | Customer Two
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	14.00%
Consolidated Accounts Receivable | Customer Three
Concentration Risk [Line Items]
Concentration risk, percentage		11.00%
Trade Accounts Payables | Supplier Concentration Risk
Concentration Risk [Line Items]
Concentration risk, percentage		12.00%
Number of supplier | Supplier	0	1
Trade Accounts Payables | Supplier Concentration Risk | Maximum
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%

Basis of Presentation and Su_13

Basis of Presentation and Summary of Significant Accounting Policies - Foreign Currency Transactions and Translation (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Foreign Currency Translation [Line Items]
Foreign currency exchange loss, net	$ (1,919,000)	$ (4,033,000)	$ (1,151,000)
Minimum
Foreign Currency Translation [Line Items]
Forward contracts terms	30 days
Maximum
Foreign Currency Translation [Line Items]
Forward contracts terms	60 days
Forward Contracts
Foreign Currency Translation [Line Items]
Notional amounts of pen forward contracts	$ 78,800,000	$ 58,400,000

Basis of Presentation and Su_14

Basis of Presentation and Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Detail) - ASU 2016-02 $ in Millions	Dec. 31, 2018USD ($)
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease, right-of-use assets	$ 40
Operating lease, liabilities	$ 50

Net Income (Loss) Per Share A_3

Net Income (Loss) Per Share Attributable to Common Stockholders - Computation of Numerators and Denominators of Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 29, 2016USD ($)	Dec. 31, 2018USD ($)Class$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)Class$ / sharesshares	Sep. 30, 2016Class
Earnings Per Share
Number of classes of common stock | Class		2		2	2
Numerator:
Net income | $		$ 88,140	$ 50,798	$ 20,482
Repurchase of convertible preferred stock | $	$ (47,200)			(47,209)
Net income (loss) attributable to common stockholders | $		$ 88,140	$ 50,798	$ (26,727)
Denominator:
Weighted-average shares outstanding—basic		42,442	40,262	18,280
Effect of dilutive securities:
Options to purchase common stock		2,845	3,415
Employee stock purchase plan shares		251	268
Restricted stock		255	111
Weighted-average shares outstanding—diluted		45,793	44,056	18,280
Basic EPS | $ / shares		$ 2.08	$ 1.26	$ (1.46)
Diluted EPS | $ / shares		$ 1.92	$ 1.15	$ (1.46)
Anti-dilutive equity awards under stock-based award plans excluded from the determination of diluted EPS		472	1,246	6,069

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Classes of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 48,703	$ 26,308
Less: Accumulated depreciation	(15,657)	(8,903)
Property and equipment, Net	33,046	17,405
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	11,806	4,520
Purchased Software
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	9,777	7,735
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	8,175	4,306
Construction in Progress
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	252	102
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, Gross	$ 18,693	$ 9,645

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 8,100	$ 5,300	$ 2,400
Property and equipment	33,046	17,405
Accumulated depreciation	15,657	8,903
Capitalized Software Development
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 400	400	$ 400
Installment payment period	4 years
Property and equipment	$ 1,800	1,800
Accumulated depreciation	$ 1,600	$ 1,200

Capitalized Software Developm_3

Capitalized Software Development Costs (Detail) - Capitalized Software Development - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Capitalized Computer Software Net [Line Items]
Capitalized software development costs, gross	$ 11,487	$ 6,133
Less: Accumulated amortization	(3,386)	(1,734)
Capitalized software development costs, net	$ 8,101	$ 4,399

Capitalized Software Developm_4

Capitalized Software Development Costs - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Capitalized Computer Software Net [Line Items]
Software not yet placed into service, included in other assets, non-current	$ 19,843	$ 10,587
Capitalized Software Development
Capitalized Computer Software Net [Line Items]
Software development costs	6,900	3,500	$ 2,400
Amortization expenses	3,200	$ 1,900	$ 1,400
Estimated amortization in 2019	4,200
Estimated amortization in 2020	2,000
Software not yet placed into service, included in other assets, non-current	$ 1,900

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) $ / shares in Units, $ in Millions	Sep. 26, 2016USD ($)$ / sharesshares	Sep. 02, 2016	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Reverse stock split ratio		0.3333
IPO
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
IPO price (in dollars per share) | $ / shares	$ 18
Class B common stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Stock issued in net exercise of warrants (in shares)	448,545
Convertible Preferred Stock Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Outstanding warrants converted during the period (in shares)	1,382,505
Aggregate fair value of warrants | $	$ 12.6
Class B Common Stock Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants issued in conversion (in shares)	460,834
Stock issued in net exercise of warrants (in shares)	448,545
Money Market Funds | Level 1
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash equivalents | $			$ 172.1	$ 5

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Changes in the Fair Value of Preferred Stock Warrants (Detail) - Convertible Preferred Stock Warrants $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 6,927
Change in value of preferred stock warrants recorded in other expense, net	9,458
Reclassification to convertible preferred stock upon net exercise of Series Seed warrant in February 2016	(3,789)
Conversion of preferred stock warrants to common stock warrants upon the closing of the Company’s IPO on September 26, 2016	$ (12,596)

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	Oct. 26, 2018	Sep. 26, 2016	Dec. 31, 2018
Second A&R Credit Facility
Long-term debt:
Credit facility, borrowing capacity	$ 150,000,000
Credit Facility additional borrowing amount	$ 100,000,000
Outstanding debt balance			$ 0
Credit facility available amount			$ 143,300,000
Credit facility, maturity	May 9, 2022
Second A&R Credit Facility | Minimum
Long-term debt:
Credit Facility, fee for undrawn amounts	0.225%
Consolidated fixed charge coverage ratio	300.00%
Second A&R Credit Facility | Maximum
Long-term debt:
Credit Facility, fee for undrawn amounts	0.40%
Consolidated fixed charge coverage ratio	350.00%
Second A&R Credit Facility | Federal Funds Rate
Long-term debt:
Credit Facility, interest rate above base rate	0.50%
Second A&R Credit Facility | LIBOR
Long-term debt:
Credit Facility, interest rate above base rate	2.00%
Second A&R Credit Facility | LIBOR | Minimum
Long-term debt:
Credit Facility, interest rate above base rate	1.25%
Second A&R Credit Facility | LIBOR | Maximum
Long-term debt:
Credit Facility, interest rate above base rate	2.25%
Second A&R Credit Facility | Base Rate | Minimum
Long-term debt:
Credit Facility, interest rate above base rate	0.25%
Second A&R Credit Facility | Base Rate | Maximum
Long-term debt:
Credit Facility, interest rate above base rate	1.25%
Swingline Borrowings | Second A&R Credit Facility
Long-term debt:
Credit facility, borrowing capacity	$ 20,000,000
Letter of Credit | Second A&R Credit Facility
Long-term debt:
Credit facility, borrowing capacity	$ 15,000,000
Prior Debt Facility
Long-term debt:
Liquidation fee upon completion of IPO		$ 800,000

Capitalization - Common and Pre

Capitalization - Common and Preferred Stock (Detail)	1 Months Ended	4 Months Ended	12 Months Ended
	Sep. 30, 2016ClassVote	Dec. 31, 2016Classshares	Dec. 31, 2018Class$ / sharesshares	Dec. 31, 2017$ / sharesshares
Class of Stock [Line Items]
Number of classes of common stock | Class	2	2	2
Conversion of convertible Class B common stock to Class A common stock		4,200,000	3,400,000	21,800,000
Common stock, authorized shares			1,095,000,000
Common stock, par value (in dollars per share) | $ / shares			$ 0.000001	$ 0.000001
Preferred stock, authorized shares			100,000,000	100,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.000001	$ 0.000001
Preferred stock, shares outstanding			0	0
Class B common stock
Class of Stock [Line Items]
Number of votes per share of common stock | Vote	10
Ratio for conversion into Class A common stock	1
Threshold percentage of aggregate outstanding common shares to trigger conversion	10.00%
Threshold percentage vote or written consent of holders to trigger conversion	66.67%
Common stock, authorized shares			95,000,000	95,000,000
Class A common stock
Class of Stock [Line Items]
Number of votes per share of common stock | Vote	1
Common stock, authorized shares			1,000,000,000	1,000,000,000

Capitalization - Initial Public

Capitalization - Initial Public Offering (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 26, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Net proceeds from the offering		$ 78,120
Class B common stock
Class of Stock [Line Items]
Stock issued in net exercise of warrants (in shares)	448,545
Number of shares issued for automatic conversion of convertible preferred stock	22,078,637
Number of common stock shares issued for each share of convertible preferred stock	0.3333
IPO
Class of Stock [Line Items]
IPO price (in dollars per share)	$ 18
Net proceeds from the offering	$ 78,100
Underwriting commissions and discounts	5,900
Offering costs—IPO	4,500
Offering costs reclassified from other assets to additional paid-in capital	$ 4,500
IPO | Class A common stock
Class of Stock [Line Items]
Number of shares issued and sold by the Company	4,666,667
Number of shares sold by selling shareholders	700,000
Convertible Preferred Stock Warrants
Class of Stock [Line Items]
Outstanding warrants converted during the period (in shares)	1,382,505
Class B Common Stock Warrants
Class of Stock [Line Items]
Warrants issued in conversion (in shares)	460,834
Stock issued in net exercise of warrants (in shares)	448,545

Capitalization - Secondary Offe

Capitalization - Secondary Offering (Detail) - Class A common stock - Secondary Offering - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Jun. 30, 2017	Mar. 31, 2017
Class of Stock [Line Items]
Number of shares sold by selling shareholders net of option granted to underwriters	4,846,600	7,281,789
Secondary offering price	$ 52	$ 35.50
Additional shares sold under option granted to underwriters	530,148	949,798
Offering costs incurred	$ 0.6	$ 0.9

Capitalization - Convertible Pr

Capitalization - Convertible Preferred Stock Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands		Sep. 26, 2016	Feb. 29, 2016	Dec. 31, 2016
Convertible Preferred Stock
Proceeds from issuance of shares				$ 60,000
Proceeds from stock issuance used to repurchase other shares			$ 54,000	54,000
Preferred stock repurchase price per share, as a percentage of the Series C offering price			80.00%
Repurchase price of convertible preferred stock in excess of carrying value			$ 47,200	$ 47,209
Common Stock
Convertible Preferred Stock
Stock issued in net exercise of warrants (in shares)	[1]			449,000
Share price (in dollars per share)			$ 12.51
Number of common stock shares repurchased			188,786	189,000	[1]
Additional Paid-In Capital
Convertible Preferred Stock
Repurchase price of convertible preferred stock in excess of carrying value			$ 1,200	$ 1,168
Retained Earnings (Accumulated Deficit)
Convertible Preferred Stock
Repurchase price of convertible preferred stock in excess of carrying value			$ 46,000	46,041
Series Seed preferred stock warrants
Convertible Preferred Stock
Stock issued in net exercise of warrants (in shares)			788,755
Repurchase of stock that was issued in net exercise of warrants (in shares)			614,052
Warrants net exercised (in shares)			808,135
Series C convertible preferred stock
Convertible Preferred Stock
Issuance of stock (in shares)			11,500,587
Proceeds from issuance of shares			$ 60,000
Share price (in dollars per share)			$ 5.22
Series Seed convertible preferred stock
Convertible Preferred Stock
Number of preferred stock shares repurchased			3,897,928
Stock issued in net exercise of warrants (in shares)			788,755
Repurchase of stock that was issued in net exercise of warrants (in shares)			614,052
Series A convertible preferred stock
Convertible Preferred Stock
Number of preferred stock shares repurchased			8,485,350
Convertible Preferred Stock
Convertible Preferred Stock
Repurchase price including legal costs			$ 51,800
Carrying value of stock at the date of repurchase			$ 4,600
Repurchase price of convertible preferred stock in excess of carrying value				$ 4,623
Class B common stock
Convertible Preferred Stock
Stock issued in net exercise of warrants (in shares)		448,545
Number of shares issued for automatic conversion of convertible preferred stock		22,078,637
Number of common stock shares issued for each share of convertible preferred stock		0.3333
[1]	Refer to Note 8-Capitalization for discussion of the establishment of the Company’s two classes of common stock and the reclassification of its common stock into Class B common stock prior to the Company’s initial public offering (“IPO”) in September 2016.

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense by Operating Expense Category (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	$ 42,210	$ 21,317	$ 5,056
Platform operations
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	4,463	2,674	756
Sales and marketing
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	11,306	6,261	1,707
Technology and development
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	13,855	6,661	1,513
General and administrative
Stock-based compensation expense, by operating expense category
Stock-based compensation expense	$ 12,586	$ 5,721	$ 1,080

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Award Plans - Additional Information (Detail) - 2016 Incentive Award Plan - shares shares in Millions	Jan. 01, 2019	Dec. 31, 2018
Stock-Based Compensation
Shares remained available for grant		3.6
Maximum annual increase in shares available for issuance, percentage of outstanding shares		4.00%
Stock incentive plans, vesting period		4 years
Stock incentive plans, expiration period		10 years
Restricted Stock
Stock-Based Compensation
Stock incentive plans, vesting period		4 years
Subsequent Event
Stock-Based Compensation
Number of additional shares authorized for grant	1.8

Stock-Based Compensation - St_2

Stock-Based Compensation - Stock Options (Detail) - Stock Options $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Shares Under Option
Outstanding at the beginning of the period (in shares) | shares	4,745
Granted (in shares) | shares	1,453
Exercised (in shares) | shares	(1,446)
Cancelled (in shares) | shares	(122)
Outstanding at the end of the period (in shares) | shares	4,630
Exercisable as of December 31, 2018 | shares	1,974
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 17.96
Granted (in dollars per share) | $ / shares	70.53
Exercised (in dollars per share) | $ / shares	6.93
Cancelled (in dollars per share) | $ / shares	50.96
Outstanding at the end of the period (in dollars per share) | $ / shares	37.03
Exercisable as of December 31, 2018 | $ / shares	$ 15.94
Stock Options, additional disclosures
Weighted-Average Contractual Life, outstanding	7 years 7 months 6 days
Weighted-Average Contractual Life, exercisable	6 years 2 months 12 days
Aggregate Intrinsic Value, Outstanding | $	$ 371,550
Aggregate Intrinsic Value, Exercisable | $	$ 197,629

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Weighted-Average Assumptions Used to Value Options Granted to Employees (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted average assumptions used to value options granted to employees
Estimated dividend yield	0.00%
Stock Options
Weighted average assumptions used to value options granted to employees
Expected term (years)	6 years	6 years	6 years
Volatility (as a percent)	48.20%	52.60%	58.10%
Risk-free interest rate (as a percent)	2.85%	2.03%	1.62%
Estimated dividend yield	0.00%	0.00%	0.00%

Stock-Based Compensation - St_3

Stock-Based Compensation - Stock Options - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Options, additional disclosures
Stock-based compensation expense	$ 42,210	$ 21,317	$ 5,056
Stock Options
Weighted average assumptions used to value options granted to employees
Weighted average grant date fair value per share	$ 35.08	$ 22.48	$ 11.61
Total intrinsic value of options exercised	$ 119,000	$ 84,800	$ 13,700
Stock Options, additional disclosures
Stock-based compensation expense	17,700	$ 7,900	$ 1,700
Unrecognized employee stock-based compensation	$ 68,600
Unrecognized employee stock-based compensation, recognition period	2 years 8 months 12 days

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Nonvested Restricted Stock Activity (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Unvested, Shares, beginning balance | shares	418
Granted, Shares | shares	239
Vested, Shares | shares	(134)
Forfeited, Shares | shares	(5)
Unvested, Shares, ending balance | shares	518
Unvested, Weighted-Average Grant Date Fair Value Per Share, beginning balance | $ / shares	$ 39.95
Granted, Weighted-Average Grant Date Fair Value Per Share | $ / shares	96.21
Vested, Weighted-Average Grant Date Fair Value Per Share | $ / shares	41.32
Forfeited, Weighted-Average Grant Date Fair Value Per Share | $ / shares	83.10
Unvested, Weighted-Average Grant Date Fair Value Per Share, ending balance | $ / shares	$ 65.14

Stock-Based Compensation - Rest

Stock-Based Compensation - Restricted Stock - Additional Information (Detail) - USD ($) $ in Thousands	Nov. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Stock-based compensation expense		$ 42,210	$ 21,317	$ 5,056
Performance-based restricted stock award granted		239,000
Restricted Stock
Stock-Based Compensation
Stock-based compensation expense		$ 7,400	$ 2,700	$ 100
Unrecognized employee stock-based compensation		$ 30,800
Weighted-average recognition period employee stock based compensation		2 years 8 months 12 days
PSUs
Stock-Based Compensation
Performance-based restricted stock award granted	24,000

Stock-Based Compensation - ESPP

Stock-Based Compensation - ESPP - Additional Information (Detail) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Stock-Based Compensation
Stock-based compensation expense		$ 42,210	$ 21,317	$ 5,056
ESPP
Stock-Based Compensation
Shares remained available for grant		200,000
Maximum annual increase in shares available for grant (in shares)		800,000
Maximum employee payroll deduction (as a percent)		100.00%
Maximum offering period		2 years
Period between purchases		6 months
Price of ESPP shares as percentage of market price		85.00%
Holding period for purchases after the first offering period				6 months
Holding period for purchases during the first offering period				12 months
Stock-based compensation expense		$ 17,100	$ 10,700	$ 3,300
Unrecognized employee stock-based compensation		$ 8,800
Unrecognized employee stock-based compensation, recognition period		10 months 24 days
ESPP | Subsequent Event
Stock-Based Compensation
Number of additional shares authorized for grant	400,000
ESPP | Common Stock
Stock-Based Compensation
Maximum annual increase in shares available for issuance, percentage of outstanding shares		1.00%
ESPP | Class A common stock
Stock-Based Compensation
Stock available for issuance (in shares)					800,000

Stock-Based Compensation - ES_2

Stock-Based Compensation - ESPP (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-average assumptions used to estimate the fair value of ESPP shares
Estimated dividend yield	0.00%
ESPP
Weighted-average assumptions used to estimate the fair value of ESPP shares
Expected term (years)	9 months 18 days	10 months 24 days	9 months 18 days
Volatility (as a percent)	46.20%	45.90%	48.90%
Risk-free interest rate (as a percent)	2.32%	1.22%	0.69%
Estimated dividend yield	0.00%	0.00%	0.00%

Income Taxes - Domestic and For

Income Taxes - Domestic and Foreign Components of Income (Loss) before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income before income taxes	$ 105,737	$ 63,625	$ 43,834
UNITED STATES
Income before income taxes	115,706	66,148	45,904
International
Income before income taxes	$ (9,969)	$ (2,523)	$ (2,070)

Income Taxes - Components of Pr

Income Taxes - Components of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ 11,683	$ 9,944	$ 18,300
State and local	9,295	3,906	5,595
Foreign	1,720	558	64
Total current provision	22,698	14,408	23,959
Deferred:
Federal	(918)	(172)	(68)
State and local	(2,615)	(1,382)	(539)
Foreign	(1,568)	(27)
Total deferred provision	(5,101)	(1,581)	(607)
Total provision for income taxes	$ 17,597	$ 12,827	$ 23,352

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Tax Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	21.00%	35.00%	35.00%
State and local income taxes, net of federal benefit	5.00%	2.60%	7.50%
Foreign income at other than U.S. rates	2.10%	2.70%	1.10%
Stock-based compensation	(7.60%)	(19.00%)	3.80%
Meals and entertainment	0.50%	0.40%	0.30%
Change in value of preferred stock warrant liabilities			7.60%
Research and development credit	(3.90%)	(2.80%)	(3.10%)
Federal deferred tax asset revaluation		0.90%
Other permanent items	(0.40%)	0.90%	0.40%
Change in valuation allowance		(0.50%)	0.70%
Effective income tax rate	16.70%	20.20%	53.30%

Income Taxes - Tax Effects of T

Income Taxes - Tax Effects of Temporary Differences that Give Rise to Significant Portion of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets (liabilities):
Reserves and allowances	$ 2,465	$ 1,804
Accrued expenses	4,193	1,810
Net operating losses	854
Research and development tax credit	1,367	774
Stock-based compensation	4,726	1,722
Deferred rent	1,425	895
Prepaid expenses	(559)	(580)
Property and equipment	(4,141)	(2,026)
Capitalized software development costs	(1,874)	(1,116)
Other	4	76
Total deferred tax assets, net	$ 8,460	$ 3,359

Income Tax - Additional Informa

Income Tax - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Research and development tax credits	$ 1,367	$ 774
Cash paid for income taxes	17,287	19,163	$ 16,740
Gross unrecognized tax benefits	4,330	3,101	$ 1,007
Expected reduction in unrecognized tax benefits	200
Other Liabilities, Non current
Gross unrecognized tax benefits	4,300	$ 3,100
UNITED KINGDOM
Operating loss carryforwards	900
Research and development tax credits	200
UNITED STATES
Research and development tax credits	2,400
International
Unremitted earnings of subsidiaries, foreign	14,100
Cash paid for income taxes	$ 0

Income Taxes - Schedule of Chan

Income Taxes - Schedule of Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning balance	$ 3,101	$ 1,007
Increases related to prior year tax positions		123	$ 402
Decreases related to prior year tax positions	(270)
Increases related to current year tax positions	1,499	1,971	605
Ending balance	$ 4,330	$ 3,101	$ 1,007

Segment and Geographical Inform

Segment and Geographical Information - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)Segment	Dec. 31, 2017USD ($)
Segment Reporting Information [Line Items]
Operating segment | Segment	1
Property and equipment, net	$ 33,046	$ 17,405
International
Segment Reporting Information [Line Items]
Property and equipment, net	$ 7,200	$ 3,600

Segment and Geographical Info_2

Segment and Geographical Information - Gross Billings, Based on Billing Address of Clients or Client Affiliates (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross Billings	$ 2,285,013	$ 1,491,742	$ 990,561
UNITED STATES
Segment Reporting Information [Line Items]
Gross Billings	1,937,074	1,270,116	868,877
International
Segment Reporting Information [Line Items]
Gross Billings	$ 347,939	$ 221,626	$ 121,684

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Minimum Lease Commitments under Non-cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)	[1]
Non-cancelable minimum lease commitments
2,019	$ 13,419
2,020	18,746
2,021	29,054
2,022	23,786
2,023	20,902
Thereafter	119,109
Total	$ 225,016
[1]	Includes non-cancelable minimum lease payments of $183.5 million for leases executed in 2018 that the Company plans to occupy in 2019.

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Minimum Lease Commitments under Non-cancelable Operating Leases (Parenthetical) (Detail) $ in Millions	Dec. 31, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
Non-cancelable minimum lease payments for leases	$ 183.5

Commitments and Contingencies_3

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies
Rent expense		$ 10,900,000	$ 8,200,000	$ 4,800,000
Indemnifications
Commitments and Contingencies
Recorded obligation		$ 0	$ 0
Subsequent Event
Commitments and Contingencies
Hosting services arrangement commitment	$ 200,000,000
Agreement completion date	2024-04

Commitments and Contingencies_4

Commitments and Contingencies - Schedule of Purchase Obligations (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Purchase obligations
2,019	$ 33,352
2,020	516
2,021	230
2,022	230
Total	$ 34,328