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Employee Benefit Plans (Tables)

3 Months Ended

Mar. 31, 2019

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

The following is a summary of stock option activity for the current year period:

	Stock Options
	Number of Shares			Weighted Average Exercise Price		Aggregate Intrinsic Value (in thousands)		Weighted Average Remaining Contractual Term (in years)
Outstanding at December 31, 2018		18,452,375		$	14.39	$	8,482		7.5
Granted		2,225,766		$	12.35
Exercised		(1,327,784	)	$	10.86
Forfeited		(447,402	)	$	14.33
Expired		(886,650	)	$	15.99
Outstanding, vested and expected to vest at March 31, 2019		18,016,305		$	14.32	$	70,104		7.7
Vested and exercisable at March 31, 2019		9,234,768		$	14.72	$	32,262		6.7

Summary of Estimated Fair Value of Stock Options on Grant Date

The Company used the Black-Scholes-Merton option pricing model to estimate the fair value of stock options granted using the following assumptions

	Three Months Ended March 31,
	2019	2018
Expected life (in years)	5.9	5.8
Risk-free interest rate	2.5%	2.7%
Volatility	46%	43%
Dividend yield	—%	—%
Fair value of common stock	$12.35	$13.20

Summary of Stock-based Compensation Expense Recognized in Financial Statements

Stock-based compensation expense recognized in the condensed consolidated financial statements is as follows:

	Three Months Ended March 31,
(in thousands)	2019		2018
Cost of revenue	$	1,096	$	658
Research and development		4,766		3,447
Sales and marketing		2,780		768
General and administrative		6,469		3,667
Stock-based compensation expense, net of amounts capitalized		15,111		8,540
Capitalized stock-based compensation expense		953		328
Stock-based compensation expense	$	16,064	$	8,868

Summary of Unamortized Stock-based Compensation

As of March 31, 2019, unamortized stock-based compensation was as follows:

	Unrecognized stock-based compensation (in thousands)		Weighted average vesting period (in years)
Stock options	$	49,343		2.3
Restricted stock units (service-based)		55,568		2.9
Restricted stock units (performance-based)⁽¹⁾		19,784		1.4
ESPP		3,932		1.6
Total unrecognized stock-based compensation	$	128,627

(1)

The Performance Vesting Condition occurred upon the effectiveness of the registration statement for the Company's IPO, which was September 25, 2018. The remaining unrecognized stock-based compensation expense is recognized on an accelerated basis over the weighted-average remaining requisite service period.

Restricted Stock Units

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Restricted Stock Awards and Restricted Stock Units

As of March 31, 2019, included in other current assets is $6.8 million of proceeds receivable from stock option exercises. The proceeds receivable outstanding at the end of the first quarter were fully collected on April 2, 2019.

The following is a summary of restricted stock units for the current year period:

	Restricted Stock Units
	Number of Shares			Weighted Average Grant-Date Fair Value		Weighted Average Remaining Contractual Term (in years)
Unvested at December 31, 2018		5,752,046		$	12.77		1.2
Granted		3,820,944		$	14.01
Vested		(898,023	)	$	12.69
Forfeited		(316,238	)	$	13.15
Unvested at March 31, 2019		8,358,729		$	13.33		1.3