Stock-Based Compensation and Employee Benefit Plan (Tables)

6 Months Ended

Jun. 30, 2019

Compensation Related Costs [Abstract]

Stock Option and RSU Activity

A summary of stock option activity under our stock plans during the six months ended June 30, 2019 is as follows:

	Outstanding Options
	Number of Shares		Weighted Average Exercise Price		Remaining Contractual Life (Years)	Aggregate Intrinsic Value
						(in thousands)
Balances at December 31, 2018	14,558,420		$	25.93	6.78	$	3,084
Granted	981,105		11.44
Exercised	(328,026	)	4.28
Cancelled	(298,490	)	25.37
Balances at June 30, 2019	14,913,009		25.47		6.53	2,557
Vested and expected to vest at June 30, 2019	14,541,060		25.64		6.47	2,523
Exercisable at June 30, 2019	8,716,781		28.77		4.96	1,764

RSU Activity and Related Information

A summary of our restricted stock units ("RSUs") activity and related information during the six months ended June 30, 2019 is as follows:

	Number of Awards Outstanding		Weighted Average Grant Date Fair Value
Unvested Balance at December 31, 2018	16,784,800		$	18.74
Granted	2,966,254		12.36
Vested	(3,504,098	)	20.51
Forfeited	(316,053	)	17.38
Unvested Balance at June 30, 2019	15,930,903		17.19

Weighted-Average Valuation Assumptions

We used the following weighted-average assumptions in applying the Black-Scholes valuation model:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	2.4% - 2.5%	2.7% - 2.8%	2.4% - 2.6%	2.5% - 2.8%
Expected term (years)	6.4 - 6.7	6.2 - 6.7	6.4 - 6.7	6.2 - 6.7
Expected dividend yield	—	—	—	—
Expected volatility	47.5%	54.6%	47.5% - 50.2%	54.6% - 55.1%

Employee and Non-Employee Stock-Based Compensation Expense

The following table summarizes the components of stock-based compensation expense in the consolidated statements of operations (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Cost of revenue	$	10,392	$	1,971	$	24,764	$	3,869
Research and development	12,218		1,739		26,448		3,376
Sales and marketing	8,935		1,214		20,447		2,166
General and administrative	19,673		2,894		43,441		6,362
Total stock-based compensation	$	51,218	$	7,818	$	115,100	$	15,773