Stock-Based Compensation and Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2019

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



	Years Ended December 31,
	2019	2018	2017

Risk-free interest rate	1.7% - 2.6%	2.5% - 3.1%	2.0% - 2.1%
Expected term (years)	6.4 - 6.7	6.2 - 6.7	6.1 - 6.6
Expected dividend yield	—	—	—
Expected volatility	45.7% - 50.2%	52.4% - 56.1%	55.6% - 61.0%

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,
	2019		2018		2017
			As Restated		As Revised

Cost of revenue	$	45,429	$	29,680	$	6,355
Research and development	40,949		39,029		5,560
Sales and marketing	32,478		32,284		4,685
General and administrative	77,435		67,489		12,501
	$	196,291	$	168,482	$	29,101

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, 2019:



	Plan Shares Available for Grant


Balances at December 31, 2017	1,037,616
Added to plan	40,924,861
Granted	(18,075,790	)
Cancelled	1,061,054
Expired	(7,489,894	)
Balances at December 31, 2018	17,457,847
Added to plan	7,585,422
Granted	(8,176,023	)
Cancelled	2,289,290
Expired	(1,923,392	)
Balances at December 31, 2019	17,233,144

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, 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
Granted	4,956,064		5.6
Exercised	(358,564	)	4.26
Cancelled	(1,318,604	)	25.33
Balances at December 31, 2019	17,837,316		20.76		6.94	14,964
Vested and expected to vest at December 31, 2019	17,159,824		21.17		6.85	13,471
Exercisable at December 31, 2019	9,161,918		28.82		4.89	500

RSU Activity and Related Information

A summary of our RSUs activity and related information is as follows:



	Number of Awards Outstanding		Weighted Average Grant Date Fair Value

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
Granted	3,219,959		11.81
Vested	(8,921,807	)	18.03
Forfeited	(970,686	)	17.34
Unvested Balance at December 31, 2019	10,112,266		17.29

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,
	2019	2018

Risk-free interest rate	1.5% - 2.6%	2.2% - 2.7%
Expected term (years)	0.5 - 2.0	0.6 - 2.0
Expected dividend yield	—	—
Expected volatility	45.9% - 54.0%	47.0% - 52.7%