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Stock award plan (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of stock option activity
A summary of stock option activity is as follows:
 
 
Number of
options
 
Weighted-
average
exercise
price
 
Weighted-
average
remaining
contractual
term
 
Aggregate
intrinsic
value
 
 
 
 
 
 
 
 
(in thousands)
Outstanding at December 31, 2014
 
3,432,972

 
$
25.00

 
 
 
 

Granted
 
2,201,800

 
$
50.81

 
 
 
 

Exercised
 
(656,248
)
 
$
13.27

 
 
 
 

Forfeited
 
(152,047
)
 
$
49.58

 
 
 
 

Outstanding at December 31, 2015
 
4,826,477

 
$
37.20

 
 
 
 

Granted
 
1,500,645

 
$
27.90

 
 
 
 

Exercised
 
(89,216
)
 
$
10.85

 
 
 
 

Forfeited
 
(383,590
)
 
$
47.42

 
 
 
 

Outstanding at December 31, 2016
 
5,854,316

 
$
34.71

 
 
 
 

Granted
 
1,913,873

 
$
12.34

 
 
 
 

Exercised
 
(202,085
)
 
$
10.80

 
 
 
 

Forfeited
 
(1,117,462
)
 
$
33.65

 
 
 
 

Outstanding at December 31, 2017
 
6,448,642

 
$
29.00

 
7.28
 
$
13,435

Vested or expected to vest at December 31, 2017
 
2,522,747

 
$
23.29

 
8.39
 
$
6,822

Exercisable at December 31, 2017
 
3,778,544

 
$
33.19

 
6.48
 
$
6,167
Schedule of assumptions used to estimate fair values of grants made on the date of grant
The fair values of grants made in the years ended December 31, 2017, 2016 and 2015 were contemporaneously estimated on the date of grant using the following assumptions:
 
 
2017
 
2016
 
2015
Risk-free interest rate
 
1.84 - 2.45%
 
1.30 - 2.24%
 
1.48 - 2.18%
Expected volatility
 
76 - 81%
 
67 - 78%
 
67 - 69%
Expected term
 
5.04 - 10.00 years
 
5.05 - 10.00 years
 
5.50 - 9.12 years
Summary of information on the Company's restricted stock
The following table summarizes information on the Company’s restricted stock awards and units:
 
 
Restricted Stock Awards and Units
 
 
Number of
Shares
 
Weighted
Average
Grant
Date Fair
Value
Unvested at December 31, 2016
 
271,651

 
$
19.76

Granted
 
365,194

 
$
11.67

Vested
 
(180,861
)
 
$
14.19

Forfeited
 
(62,973
)
 
$
14.18

Unvested at December 31, 2017
 
393,011

 
$
15.64
Schedule of share-based compensation expense recorded in the statement of operations
The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows:
 
 
Year ended December 31,
 
 
2017
 
2016
 
2015
Research and development
 
$
15,456

 
$
16,812

 
$
16,138

Selling, general and administrative
 
15,103

 
18,197

 
17,841

Total
 
$
30,559

 
$
35,009

 
$
33,979