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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Stock-Based Compensation Expense by Operating Expense Category

Total stock-based compensation expense, by operating expense category, recorded in the consolidated statements of operations was as follows (in thousands):

	Year Ended December 31,
	2015		2016		2017
Platform operations	$	71	$	756	$	2,674
Sales and marketing		127		1,707		6,261
Technology and development		85		1,513		6,661
General and administrative		91		1,080		5,721
Total	$	374	$	5,056	$	21,317

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, 2016 (1)		5,429		$	4.94
Granted		1,473			44.20
Exercised		(1,932	)		1.33
Cancelled		(225	)		18.49
Outstanding as of December 31, 2017 (2)		4,745		$	17.96		7.4	$	136,085
Exercisable as of December 31, 2017		2,242		$	4.81		5.9	$	91,829

(1)	Includes options to purchase 389 and 5,040 shares of Class A and Class B common stock, respectively.

(2)	Includes options to purchase 1,780 and 2,965 shares of Class A and Class B common stock, respectively.

Schedule of Weighted-Average Assumptions Used to Value Options Granted to Employees

The fair value of options on the date of grant is 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,
	2015			2016			2017
Expected term (years)		6.0			6.0			6.0
Expected volatility		64.5	%		58.1	%		52.6	%
Risk-free interest rate		1.62	%		1.62	%		2.03	%
Estimated dividend yield		—	%		—	%		—	%

Schedule 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, 2016		193		$	29.65
Granted		307			43.76
Vested		(54	)		30.06
Forfeited		(28	)		29.97
Unvested as of December 31, 2017		418		$	39.95

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,
	2016			2017
Expected term (years)		0.8			0.9
Expected volatility		48.9	%		45.9	%
Risk-free interest rate		0.69	%		1.22	%
Estimated dividend yield		—	%		—	%