Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Stock-Based Compensation Expense Related to Grants of Employee Stock Options, Restricted Stock Units (ZSUs) and Performance-Based Awards

We recorded stock-based compensation expense related to grants of employee stock options, restricted stock units (“ZSUs”) and performance-based awards in our consolidated statements of operations as follows (in thousands):

	Three Months Ended March 31,
	2019		2018
Cost of revenue	$	284	$	431
Research and development		11,715		8,625
Sales and marketing		2,487		1,836
General and administrative		4,287		3,221
Total stock-based compensation expense	$	18,773	$	14,113

Schedule of Share Based Compensation Stock Option Activity

The following table shows stock option activity for the three months ended March 31, 2019 (in thousands, except weighted-average exercise price and weighted-average contractual term):

	Outstanding Options
				Weighted-		Aggregate		Weighted-
				Average		Intrinsic Value of		Average
				Exercise		Stock Options		Contractual Term
	Stock Options			Price		Outstanding		(in years)
Balance as of December 31, 2018		36,185		$	2.35	$	57,510		6.23
Granted		4,647			5.37
Forfeited, expired and cancelled		—			2.80
Exercised		(3,835	)		0.37
Balance as of March 31, 2019		36,997		$	2.93	$	88,970		7.06

Weighted-Average Grant Date Fair Value of Stock Options and Related Assumptions

The following table presents the weighted-average grant date fair value and the related assumptions used to estimate the fair value of our stock options:

	Three Months Ended March 31, 2019
Expected term, in years		6
Risk-free interest rates		2.53	%
Expected volatility		43	%
Dividend yield		—
Weighted-average estimated fair value of options granted	$	2.41

Schedule of Share Based Compensation Restricted Stock Units Award Activity

The following table shows a summary of ZSU activity for the three months ended March 31, 2019 (in thousands, except weighted-average grant date fair value):

	Outstanding ZSUs
				Weighted-
				Average Grant Date		Aggregate
				Fair Value		Intrinsic Value of
	Shares			(per share)		Unvested ZSUs
Unvested as of December 31, 2018		52,482		$	3.49	$	206,254
Granted		3,503			5.18
Vested		(5,717	)		3.24
Forfeited		(1,051	)		2.77
Unvested as of March 31, 2019		49,217		$	3.66	$	262,332