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Employee Stock Benefit Plans (Tables)
 12 Months Ended
Dec. 31, 2017
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,
 
2017
 
2016
 
2015
Expected term (years)
6.02

 
5.95

 
6.08

Volatility
57
%
 
55
%
 
55
%
Expected dividend yield
0
%
 
0
%
 
0
%
Risk-free interest rates
2.01
%
 
1.91
%
 
1.72
%
Summary of stock option activity
A summary of stock option activity for the year ended December 31, 2017 is as follows (in thousands, except for 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, 2016
6,374

 
$
31.60

 
 
 
 

Granted
2,094

 
42.82

 
 
 
 

Exercised
(1,385
)
 
12.62

 
 
 
 

Canceled
(1,340
)
 
41.22

 
 
 
 

Expired
(95
)
 
57.56

 
 
 
 

Options outstanding at December 31, 2017
5,648

 
$
37.71

 
7.03
 
$
23,150

Options exercisable at December 31, 2017
2,934

 
$
33.23

 
5.44
 
$
21,365
Summary of restricted stock units activity
A summary of RSU activity during the year ended December 31, 2017 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, 2016
57

 
$
33.03

Granted
137

 
38.62

Vested
(14
)
 
33.03

Forfeited
(33
)
 
40.04

RSUs Outstanding at December 31, 2017
147

 
$
36.69
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,
 
2017
 
2016
 
2015
Research and development
$
14,699

 
$
11,190

 
$
7,864

General and administrative
20,304

 
14,868

 
6,870

Share-based compensation expense included in operating expenses
$
35,003

 
$
26,058

 
$
14,734
Summary of performance-based units activity
A summary of PSU activity during the year ended December 31, 2017 is as follows (in thousands, except for per share amounts):
 
PSUs
 
Weighted-Average Grant Date Fair Value (in dollars per share)

PSUs Outstanding at December 31, 2016
25

 
$
49.59

Granted
—

 
—

Vested
—

 
—

Forfeited
(25
)
 
49.59

PSUs Outstanding at December 31, 2017
—

 
$
—