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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Share-based Compensation [Abstract]  
Summary of Fair Value Assumptions for Stock Options The following table summarizes the grant date fair value and assumptions used in determining the fair value of all stock options granted, including grants of inducement options, during the years ended December 31, 2018, 2017 and 2016.
 
2018
 
2017
 
2016
Volatility
66% - 68%
 
71% - 79%
 
74% - 77%
Risk-free interest rate
2.25% - 2.96%
 
1.73% - 2.13%
 
1.00% - 1.90%
Dividend yield
0.0%
 
0.0%
 
0.0%
Expected option term (in years)
5.09
 
6.25
 
6.25
Weighted average fair value of stock options granted
$16.03
 
$10.52
 
$8.77
Summary of Stock Option Activity The following table summarizes stock option activity for stock options granted for the years ended December 31, 2018, 2017 and 2016 as follows:
 
Number of
Shares
 
Weighted
Average
Exercise
Price
 
Weighted
Average
Remaining
Contractual
Life in Years
 
Aggregate
Intrinsic
Value
(in '000)
Options outstanding at January 1, 2016
5,273,722

 
$
13.64

 
 
 
 

Granted
2,532,675

 
$
12.96

 
 
 
 

Exercised
(162,340
)
 
$
6.68

 
 
 
 

Forfeited and expired
(527,351
)
 
$
17.08

 
 
 
 

Options outstanding at December 31, 2016
7,116,706

 
$
13.30

 
 
 
 

Vested and expected to vest at December 31, 2016
6,850,658

 
$
13.25

 
 
 
 

Exercisable at December 31, 2016
3,113,998

 
$
11.28

 
 
 
 

Options outstanding at December 31, 2016
7,116,706

 
$
13.30

 
 
 
 

Granted
2,284,710

 
$
15.92

 
 
 
 

Exercised
(378,275
)
 
$
9.08

 
 
 
 

Forfeited and expired
(414,220
)
 
$
15.50

 
 
 
 

Options outstanding at December 31, 2017
8,608,921

 
$
14.08

 
 
 
 
Vested and expected to vest at December 31, 2017
8,325,255

 
$
14.03

 
 
 
 
Exercisable at December 31, 2017
4,229,478

 
$
12.71

 
 
 
 
Options outstanding at December 31, 2017
8,608,921

 
$
14.08

 
 
 
 
Granted
1,755,600

 
$
27.63

 
 
 
 

Exercised
(494,351
)
 
$
14.46

 
 
 
 

Forfeited and expired
(488,440
)
 
$
19.79

 
 
 
 
Options outstanding at December 31, 2018
9,381,730

 
$
16.30

 
6.69
 
$
13,052

Vested and expected to vest at December 31, 2018
8,693,635

 
$
15.90

 
6.51
 
$
12,926

Exercisable at December 31, 2018
5,649,698

 
$
13.45

 
5.58
 
$
12,181
Summary of Exercise Price and Number of Stock Options Exercisable The following table summarizes the range of exercise prices and the number of stock options outstanding and exercisable as of December 31, 2018:
Outstanding as of December 31, 2018
 
Exercisable as of
December 31, 2018
Range of
Exercise Prices
 
Number of
Options
 
Weighted
Average
Remaining
Contractual Term
(in years)
 
Weighted
Average
Exercise Price
 
Number of
Options
 
Weighted
Average
Exercise
Price
$
3.03

 
$
4.55

 
960,038

 
3.62
 
$
3.60

 
960,038

 
$
3.60

$
6.90

 
$
6.90

 
120,403

 
4.13
 
$
6.90

 
120,403

 
$
6.90

$
6.96

 
$
10.85

 
953,089

 
6.85
 
$
10.76

 
604,037

 
$
10.71

$
11.14

 
$
12.58

 
947,839

 
4.72
 
$
12.17

 
887,207

 
$
12.18

$
12.66

 
$
13.94

 
967,420

 
7.31
 
$
13.62

 
455,114

 
$
13.59

$
14.24

 
$
16.07

 
1,142,513

 
5.76
 
$
15.31

 
782,818

 
$
15.22

$
16.09

 
$
17.16

 
1,380,220

 
7.58
 
$
16.69

 
687,691

 
$
16.57

$
17.24

 
$
22.76

 
1,387,909

 
6.51
 
$
21.04

 
1,081,677

 
$
21.31

$
22.84

 
$
30.46

 
1,355,366

 
8.37
 
$
28.54

 
57,644

 
$
25.41

$
30.86

 
$
32.46

 
166,933

 
8.60
 
$
31.20

 
13,069

 
$
31.57
Summary of RSU Activity The following table summarizes RSU awards granted during the years ended December 31, 2018, 2017 and 2016:
 
 
Number of
RSUs
 
Weighted
Average
Grant Price
Outstanding at January 1, 2016
 
43,554

 
$
19.47

Granted
 
89,194

 
$
10.85

Released
 
(43,554
)
 
$
16.07

Forfeited
 
—

 
$
—

Outstanding at December 31, 2016
 
89,194

 
$
10.85

Granted
 
46,914

 
$
17.16

Released
 
(89,194
)
 
$
10.85

Forfeited
 
—

 
$
—

Outstanding at December 31, 2017
 
46,914

 
$
17.16

Granted
 
253,586

 
$
29.16

Released
 
(51,992
)
 
$
18.46

Forfeited
 
(20,682
)
 
$
29.05

Outstanding at December 31, 2018
 
227,826

 
$
29.14
Summary of Allocation of Employee Stock-Based Compensation The following table summarizes the stock-based compensation recorded in the consolidated statements of comprehensive loss related to stock options and RSUs during the years ended December 31, 2018, 2017 and 2016 (in millions):
 
2018
 
2017
 
2016
Research and development expenses
$
9.4

 
$
6.5

 
$
6.2

Selling, general and administrative expenses
16.8

 
11.6

 
11.8

Total (1)
$
26.2

 
$
18.1

 
$
18.0


_________________________________

(1)
Includes $0.0 million, $0.0 million and $1.7 million for the years ended December 31, 2018, 2017 and 2016, respectively, for the remeasurement of certain stock options and RSUs that occurred during May 2013.