Stock-Based Compensation and Employee Benefit Plan (Tables)

9 Months Ended

Sep. 30, 2019

Compensation Related Costs [Abstract]

Stock Option and RSU Activity

A summary of stock option activity under our stock plans during the nine months ended September 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
Added to plan	—
Granted	1,433,064		11.19
Exercised	(336,572	)	4.28
Cancelled	(687,050	)	24.82
Balances at September 30, 2019	14,967,862		25.06		6.28	—
Vested and expected to vest at September 30, 2019	14,632,346		25.23		6.22	—
Exercisable at September 30, 2019	9,238,860		28.75		4.77	—

RSU Activity and Related Information

A summary of our restricted stock units ("RSUs") activity and related information during the nine months ended September 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	3,052,644		12.20
Vested	(6,341,441	)	18.27
Forfeited	(839,062	)	17.58
Unvested Balance at September 30, 2019	12,656,941		17.48

Weighted-Average Valuation Assumptions

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

	Three Months Ended September 30,		Nine Months Ended September 30,
	2019	2018	2019	2018
Risk-free interest rate	1.9% - 2.0%	2.8% - 2.8%	1.9% - 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	45.7%	56.1%	45.7% - 50.2%	54.6% - 56.1%

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 September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Cost of revenue	$	6,793	$	16,156	$	31,557	$	20,025
Research and development	6,589		15,127		33,037		18,504
Sales and marketing	5,823		12,900		26,270		15,066
General and administrative	20,650		27,494		64,091		33,856
Total stock-based compensation	$	39,855	$	71,677	$	154,955	$	87,451