Stock-Based Compensation and Employee Benefit Plan (Tables)

12 Months Ended

Dec. 31, 2018

Compensation Related Costs [Abstract]

Weighted-Average Valuation Assumptions

The Company uses weighted-average assumptions in applying the Black-Scholes valuation model.

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

	Years Ended December 31,
	2018	2017	2016
Risk-free interest rate	2.49% - 3.06%	1.95% -2.08%	1.23% -1.69%
Expected term (years)	6.18—6.69	6.08—6.62	6.00—6.54
Expected dividend yield	—	—	—
Expected volatility	52.4% -56.1%	55.6% - 61.0%	59.3% - 60.9%

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):

	Years Ended December 31,
	2018		2017		2016
Cost of revenue	$	41,481	$	7,734	$	6,005
Research and development	39,030		5,560		4,686
Sales and marketing	32,284		4,684		5,600
General and administrative	67,489		12,501		11,866
	$	180,284	$	30,479	$	28,157

Stock Option and RSU Activity

The following table presents the stock activity and the total number of shares available for grant under our stock plans as of December 31, 2018:

	Plan Shares Available for Grant
Balances at December 31, 2016	2,768,450
Added to plan	647,159
Granted	(2,698,594	)
Exercised	—
Cancelled	967,760
Expired	(647,159	)
Balances at December 31, 2017	1,037,616
Added to plan	40,924,861
Granted	(18,075,790	)
Exercised	—
Cancelled	1,061,054
Expired	(7,489,894	)
Balances at December 31, 2018	17,457,847

The following table summarizes the stock option activity under our stock plans during the reporting period (in thousands), except per share amounts:

	Outstanding Options
	Number of Shares		Weighted Average Exercise Price		Remaining Contractual Life (Years)	Aggregate Intrinsic Value
						(in thousands)
Balances at December 31, 2016	10,504,750		$	23.85	6.11	$	74,717
Granted	2,146,113		30.96
Exercised	(123,153	)	3.51
Cancelled	(923,307	)	10.78
Balances at December 31, 2017	11,604,403		26.42		6.01	52,682
Granted	4,202,284		19.79
Exercised	(398,704	)	3.98
Cancelled	(849,563	)	12.51
Balances at December 31, 2018	14,558,420		25.93		6.78	3,084
Vested and expected to vest at December 31, 2018	14,133,451		26.09		6.69	3,084
Exercisable at December 31, 2018	8,282,247		27.72		5.09	3,084

RSU Activity and Related Information

A summary of the Company’s RSU activity and related information is as follows:

	Number of Awards Outstanding		Weighted Average Grant Date Fair Value
Unvested Balance at December 31, 2016	2,666,446		$	30.95
Granted	552,481		30.96
Vested	(33,896	)	30.96
Forfeited	(44,453	)	30.95
Unvested Balance at December 31, 2017	3,140,578		30.95
Granted	13,873,506		16.02
Vested	(17,793	)	19.67
Forfeited	(211,491	)	21.22
Unvested Balance at December 31, 2018	16,784,800		18.74

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

We use the Black-Scholes option pricing model to determine the fair value of shares purchased under the 2018 ESPP with the following weighted average assumptions on the date of grant:

	Year Ended December 31,
	2018
Risk-free interest rate	2.20% - 2.67%
Expected term (years)	0.56—2.05
Expected dividend yield	—
Expected volatility	47.0% - 52.7%