Stockholders' Equity and Other Employee Benefits (Tables)

12 Months Ended

Dec. 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, ZSUs and performance-based awards in our consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2019		2018		2017
Cost of revenue	$	1,471	$	1,584	$	1,838
Research and development		47,049		42,151		42,176
Sales and marketing		11,277		8,495		7,281
General and administrative		21,685		16,009		13,220
Total stock-based compensation expense	$	81,482	$	68,239	$	64,515

Schedule of Share Based Compensation Stock Option Activity

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

	Outstanding Options
	Stock Options			Weighted- Average Exercise Price		Aggregate Intrinsic Value of Stock Options Outstanding		Weighted- Average Contractual Term (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		(42	)		2.85
Exercised		(9,586	)		0.90
Balance as of December 31, 2019		31,204		$	3.24	$	89,786		7.19
As of December 31, 2019
Exercisable options		15,666		$	2.78	$	52,363		6.59
Vested and expected to vest		31,204		$	3.24	$	89,786		7.19

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

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

	Year Ended December 31,
	2019			2018			2017
Expected term, in years		6			6			6
Risk-free interest rates		2.53	%		2.14	%		2.12	%
Expected volatility		43	%		47	%		46	%
Dividend yield		—			—			—
Weighted-average estimated fair value of stock options granted    during the year	$	2.41		$	1.61		$	1.75

Schedule of Share Based Compensation Restricted Stock Units Award Activity

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

	Outstanding ZSUs
				Weighted-
				Average Grant Date		Aggregate Intrinsic
				Fair Value		Value of
	Shares			(per share)		Unvested ZSUs
Unvested as of December 31, 2018		52,482		$	3.49	$	206,254
Granted		14,086			5.61
Vested		(21,650	)		3.43
Forfeited		(4,730	)		3.81
Unvested as of December 31, 2019		40,188		$	4.23	$	245,951

Common Stock Reserved For Future Issuance

As of December 31, 2019, we had reserved shares of common stock for future issuance as follows (in thousands):

	December 31, 2019
Stock options outstanding		31,204
ZSUs outstanding		40,188
2011 Equity Incentive Plan		152,769
2011 Employee Stock Purchase Plan		122,380
Total		346,541