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Stock award plan (Tables)
12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [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, 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

 
 
 
 

Granted
 
3,181,623

 
$
26.64

 
 
 
 

Exercised
 
(633,973
)
 
$
18.61

 
 
 
 

Forfeited
 
(461,934
)
 
$
35.36

 
 
 
 

Outstanding at December 31, 2018
 
8,534,358

 
$
28.58

 

 


Granted
 
3,977,995

 
$
35.81

 
 
 
 
Exercised
 
(949,887
)
 
$
19.25

 
 
 
 
Forfeited
 
(518,527
)
 
$
35.27

 
 
 
 
Outstanding at December 31, 2019
 
11,043,939

 
$
31.67

 
7.34 years
 
$
192,350

Vested or expected to vest at December 31, 2019
 
5,370,703

 
$
31.62

 
8.83 years
 
$
88,165

Exercisable at December 31, 2019
 
5,242,221

 
$
31.37

 
5.70 years
 
$
98,781


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, 2019, 2018 and 2017 were contemporaneously estimated on the date of grant using the following assumptions:
 
 
2019
 
2018
 
2017
Risk-free interest rate
 
1.58% - 2.63%
 
2.25% - 3.10%
 
1.84% - 2.45%
Expected volatility
 
62% - 92%
 
64% - 90%
 
76% - 81%
Expected term
 
5.75 - 6.11 years
 
5.03 - 10.00 years
 
5.04 - 10.00 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, 2018
 
571,479

 
$
17.61

Granted
 
306,549

 
$
32.97

Vested
 
(181,327
)
 
$
17.56

Forfeited
 
(54,282
)
 
$
23.14

Unvested at December 31, 2019
 
642,419

 
$
24.50


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,
 
 
2019
 
2018
 
2017
Research and development
 
$
20,836

 
$
16,096

 
$
15,456

Selling, general and administrative
 
21,298

 
17,156

 
15,103

Total
 
$
42,134

 
$
33,252

 
$
30,559