Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Activity under the 2007 Plan and 2015 Plan is as follows (in thousands except per share amounts):



	Stock Options Outstanding
	Number of Shares Subject to Stock Options		Weighted– Average Exercise Price		Weighted– Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Balance—December 31, 2017	21,386		$	3.01
Granted	—		$	—
Exercised	(4,375	)	$	2.29		$	21,744
Canceled	(748	)	$	7.33
Balance—December 31, 2018	16,263		$	3.00	4.9	$	38,459
Stock options exercisable—December 31, 2018	15,131		$	2.80	4.8	$	37,707
Stock options vested and expected to vest—December 31, 2018	16,253		$	3.00	4.9	$	38,455

Schedule of restricted stock unit activity

RSU activity under the equity incentive plans is as follows:



	RSUs Outstanding		Weighted- Average Grant Date Fair Value
	(in thousands)
Unvested balance—December 31, 2017	19,188		$	9.13
Granted	14,666		5.14
Vested	(10,113	)	8.39
Forfeited or canceled	(5,365	)	7.95
Unvested balance—December 31, 2018	18,376		6.69

Schedule of stock-based compensation expense

Total stock-based compensation recognized was as follows (in thousands):



	Year Ended December 31,
	2018		2017		2016

Cost of revenue	$	7,312	$	5,312	$	4,797
Research and development	57,188		54,123		47,207
Sales and marketing	14,726		14,959		11,575
General and administrative	17,783		17,187		15,853
Total stock-based compensation expense	$	97,009	$	91,581	$	79,432

Schedule of options valuation assumptions

The fair value of the stock option awards, warrants, awards issued under the 2015 ESPP, and awards granted to employees was estimated at the date of grant using a Black-Scholes option-pricing model. The fair value of the RSUs with market conditions were estimated using a Black-Scholes option-pricing model combined with a Monte Carlo simulation model. The fair value of these awards were estimated using the following Black-Scholes assumptions:



	Year Ended December 31,
	2018	2017	2016
Employee stock options
Expected term (in years)	—	6.25	6.25
Volatility	—%	32.2%	40.7%
Risk-free interest rate	—%	2.1%	1.6%
Dividend yield	—%	—%	—%

Warrants
Expected term (in years)	—	9.5	—
Volatility	—%	32.0%	—%
Risk-free interest rate	—%	2.1%	—%
Dividend yield	—%	—%	—%

Employee stock purchase plan
Expected term (in years)	0.5	0.5	0.5
Volatility	47.8% - 60.1%	27.7% - 31.3%	30.1% - 39.0%
Risk-free interest rate	2.1% - 2.5%	1.0% - 1.4%	0.4% - 0.6%
Dividend yield	—%	—%	—%

RSUs with market conditions
Expected term (in years)	3.0	—	—
Volatility	39.3%	—%	—%
Risk-free interest rate	2.6%	—%	—%
Dividend yield	—%	—%	—%