Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

The following table summarizes our option activity as of December 31, 2019 and 2018:



	Number of Options		Weighted Average Exercise Price per Share	Weighted Average Grant Date Fair Value per Share	Weighted- Average Remaining Contractual Term	Aggregate Intrinsic Value⁽¹⁾ (in thousands)
Outstanding at December 31, 2018	50,821,132		12.16	6.59	7.1 years	220,434
Granted	7,354,268		19.58	11.35
Exercised	(6,945,306	)	6.86	4.47
Canceled/forfeited	(5,693,179	)	15.17	8.67
Outstanding at December 31, 2019	45,536,915		13.82	7.35	7.2 years	286,310
Exercisable at December 31, 2019	22,872,221		10.13	4.86	5.9 years	216,241
Expected to vest at December 31, 2019	22,664,694		17.54	9.86	8.6 years	70,069

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⁽¹⁾Aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of common stock for those options in the money as of December 31, 2019.

Restricted Common Stock Activity

The following table summarizes our restricted stock activity during the year ended December 31, 2019:



	Number of Shares		Weighted Average Grant Date Price per Share
Outstanding, non-vested at December 31, 2018	198,597		$	12.15
Issued	—		—
Vested	(162,717	)	12.15
Canceled/forfeited	(35,880	)	12.15
Outstanding, non-vested at December 31, 2019	—		—

Restricted Common Stock Units Activity

The following table summarizes our restricted stock unit activity during the year ended December 31, 2019:



	Number of Units		Weighted Average Grant Date Price per Unit
Outstanding, non-vested at December 31, 2018	458,715		$	11.93
Issued	1,283,662		19.12
Vested	(57,344	)	11.93
Canceled/forfeited	(106,413	)	20.25
Vested in prior year and settled during the period	(401,371	)	11.93
Outstanding, non-vested at December 31, 2019	1,177,249		19.01

Weighted-Average Assumptions Used to Estimate the Fair Value of Options Granted

The weighted-average assumptions used to estimate the fair value of options granted and ESPP for the years ended December 31, 2019, 2018 and 2017 are as follows:



	Weighted Average
	Years Ended December 31,
	2019			2018			2017
Options:
Risk-free interest rate	2.29		%	2.76		%	2.02		%
Expected term	6.07 years			6.27 years			6.21 years
Expected volatility	61		%	63		%	63		%
Expected dividends	—		%	—		%	—		%
Weighted average fair value per share	$	11.35		$	9.33		$	7.96

ESPP:
Risk-free interest rate	1.95		%	*			*
Expected term	0.50 years			*			*
Expected volatility	53		%	*			*
Expected dividends	—		%	*			*
Weighted average fair value per share	$	5.98		*			*

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* - Not applicable

Stock-Based Compensation Expense

The following table presents the components and classification of stock-based compensation expense for the years ended December 31, 2019, 2018 and 2017 as follows (in thousands):



	Years Ended December 31,
	2019		2018		2017
Options	$	74,780	$	63,288	$	31,724
Restricted common stock and units	5,125		9,277		8,331
Employee stock purchase plan	1,217		—		—
Total	$	81,122	$	72,565	$	40,055
Research and development	$	48,259	$	37,659	$	21,679
General and administrative	32,863		34,906		18,376
Total	$	81,122	$	72,565	$	40,055