Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 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 June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Cost of revenue	$	397	$	564	$	681	$	995
Research and development		12,238		10,363		23,953		18,988
Sales and marketing		2,920		2,214		5,407		4,050
General and administrative		5,800		4,228		10,087		7,449
Total stock-based compensation expense	$	21,355	$	17,369	$	40,128	$	31,482

Schedule of Share Based Compensation Stock Option Activity

The following table shows stock option activity for the six months ended June 30, 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		—			—
Exercised		(7,720	)		0.41
Balance as of June 30, 2019		33,112		$	3.23	$	96,299		7.57

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:

	Six Months Ended June 30, 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 six months ended June 30, 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		11,091			5.47
Vested		(12,577	)		3.42
Forfeited		(2,248	)		3.37
Unvested as of June 30, 2019		48,748		$	3.97	$	298,825