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Stock award plan (Tables)
 12 Months Ended
Dec. 31, 2016
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, 2013
 
2,095,592

 
$
20.24

 
 
 
 

Granted
 
1,639,996

 
$
31.09

 
 
 
 

Exercised
 
(108,645
)
 
$
11.00

 
 
 
 

Forfeited
 
(193,971
)
 
$
32.91

 
 
 
 

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

 
7.80 years
 
$
341

Vested or Expected to vest at December 31, 2016
 
2,792,870

 
$
35.66

 
8.36 years
 
$
249

Exercisable at December 31, 2016
 
2,903,325

 
$
33.73

 
7.22 years
 
$
72
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, 2016, 2015 and 2014 were contemporaneously estimated on the date of grant using the following assumptions:
 
 
2016
 
2015
 
2014
Risk-free interest rate
 
1.30 - 2.24%
 
1.48 - 2.18%
 
0.11 - 2.04%
Expected volatility
 
67 - 78%
 
67 - 69%
 
70 - 91%
Expected term
 
5.05 - 10.00 years
 
5.50 - 9.12 years
 
5.50 - 6.25 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, 2015
 
344,335

 
$
10.85

Granted
 
141,185

 
$
30.86

Vested
 
(163,635
)
 
$
10.85

Forfeited
 
(50,234
)
 
$
18.93

Unvested at December 31, 2016
 
271,651

 
$
19.76
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,
 
 
2016
 
2015
 
2014
Research and development
 
$
16,812

 
$
16,138

 
$
9,739

Selling, general and administrative
 
18,197

 
17,841

 
9,571

Total
 
$
35,009

 
$
33,979

 
$
19,310