Employee Stock Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Schedule of weighted-average assumptions used in estimation of grant date fair values

The weighted-average assumptions used in the Black-Scholes option-pricing model were as follows:

	Years Ended December 31,
	2019		2018		2017
Expected term (years)	6.13		6.23		6.02
Volatility	72	%	56	%	57	%
Expected dividend yield	0	%	0	%	0	%
Risk-free interest rates	2.35	%	2.68	%	2.01	%

Summary of stock option activity

A summary of stock option activity for the year ended December 31, 2019 is as follows (in thousands, except for share, per share, and weighted-average contractual life amounts):

	Shares		Weighted-Average Exercise Price (in dollars per share)		Weighted-Average Contractual Life (In Years)	Aggregate Intrinsic Value
Options outstanding at December 31, 2018	5,462,787		$	36.88
Granted	1,110,100		19.88
Exercised	(341,337	)	8.40
Canceled	(558,747	)	33.91
Expired	(838,548	)	40.59
Options outstanding at December 31, 2019	4,834,255		$	34.69	6.76	$	4,167
Options exercisable at December 31, 2019	2,951,940		$	39.49	5.56	$	3,166

Summary of restricted stock units activity

A summary of RSU activity during the year ended December 31, 2019 is as follows:

	RSUs		Weighted-Average Grant Date Fair Value (in dollars per share)
RSUs Outstanding at December 31, 2018	227,088		$	37.69
Granted	613,650		20.04
Vested	(92,031	)	35.79
Forfeited	(134,434	)	26.17
RSUs Outstanding at December 31, 2019	614,273		$	22.83

A summary of PSU activity during the twelve months ended December 31, 2019 is as follows:

	PSUs	Weighted-Average Grant Date Fair Value (in dollars per share)
PSUs Outstanding at December 31, 2018	—	$	—
Granted	79,000	19.20
Vested	—	—
Forfeited	—
PSUs Outstanding at December 31, 2019	79,000	$	19.20

Summary of stock-based compensation expense by financial statement line

The following table summarizes stock-based compensation expense by financial statement line (in thousands):

	Years Ended December 31,
	2019		2018		2017
Research and development	$	8,768	$	11,657	$	14,699
General and administrative	14,847		17,044		20,304
Share-based compensation expense included in operating expenses	$	23,615	$	28,701	$	35,003