Employee Stock Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [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,
	2018		2017		2016
Expected term (years)	6.23		6.02		5.95
Volatility	56	%	57	%	55	%
Expected dividend yield	0	%	0	%	0	%
Risk-free interest rates	2.68	%	2.01	%	1.91	%

Summary of stock option activity

A summary of stock option activity for the year ended December 31, 2018 is as follows (in thousands, except for per share and weighted-average contractual life amounts):

	Shares		Weighted-AverageExercise Price(in dollarsper share)		Weighted-AverageContractual Life(In Years)	Aggregate Intrinsic Value
Options outstanding at December 31, 2017	5,648		$	37.71
Granted	1,447		33.76
Exercised	(472	)	16.06
Canceled	(529	)	41.64
Expired	(631	)	48.68
Options outstanding at December 31, 2018	5,463		$	36.88	7.39	$	4,910
Options exercisable at December 31, 2018	3,102		$	36.53	6.43	$	4,901

Summary of restricted stock units activity

A summary of RSU activity during the year ended December 31, 2018 is as follows (in thousands, except for per share amounts):

	RSUs		Weighted-Average Grant Date Fair Value (in dollars per share)
RSUs Outstanding at December 31, 2017	147		$	36.69
Granted	221		37.83
Vested	(37	)	36.60
Forfeited	(104	)	36.98
RSUs Outstanding at December 31, 2018	227		$	37.69

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,
	2018		2017		2016
Research and development	$	11,657	$	14,699	$	11,190
General and administrative	17,044		20,304		14,868
Share-based compensation expense included in operating expenses	$	28,701	$	35,003	$	26,058