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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
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, 2017, 2016 and 2015.
 
2017
 
2016
 
2015
Volatility
71% - 79%
 
74% - 77%
 
78% - 82%
Risk-free interest rate
1.73% - 2.13%
 
1.00% - 1.90%
 
1.31% - 1.75%
Dividend yield
0.0%
 
0.0%
 
0.0%
Expected option term (in years)
6.25
 
6.25
 
6.25
Weighted average fair value of stock options granted
$10.52
 
$8.77
 
$14.20
Summary of Stock Option Activity
The following table summarizes stock option activity for stock options granted for the years ended December 31, 2017, 2016 and 2015 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, 2015
4,400,106

 
$
10.59

 
 
 
 

Granted
1,902,850

 
20.45

 
 
 
 

Exercised
(481,140
)
 
10.62

 
 
 
 

Forfeited and expired
(548,094
)
 
15.43

 
 
 
 

Options outstanding at December 31, 2015
5,273,722

 
$
13.64

 
 
 
 

Vested and expected to vest at December 31, 2015
5,059,645

 
13.46

 
 
 
 

Exercisable at December 31, 2015
1,991,141

 
8.70

 
 
 
 

Options outstanding at December 31, 2015
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

 
7.4
 
$
147,260

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

 
$
14.03

 
7.4
 
$
142,783

Exercisable at December 31, 2017
4,229,478

 
$
12.71

 
6.3
 
$
78,138
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, 2017:
Outstanding as of December 31, 2017
 
Exercisable as of
December 31, 2017
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

 
973,195

 
4.65
 
$
3.60

 
973,195

 
$
3.60

$
6.90

 
$
6.90

 
137,577

 
5.22
 
$
6.90

 
100,077

 
$
6.90

$
6.96

 
$
10.85

 
1,077,621

 
8.29
 
$
10.76

 
420,374

 
$
10.62

$
11.14

 
$
12.58

 
1,088,035

 
6.39
 
$
12.17

 
781,991

 
$
12.18

$
12.66

 
$
13.67

 
1,043,273

 
8.74
 
$
13.59

 
101,126

 
$
13.26

$
13.94

 
$
15.91

 
862,300

 
7.76
 
$
14.97

 
406,441

 
$
14.53

$
16.07

 
$
16.16

 
1,008,750

 
7.69
 
$
16.13

 
463,629

 
$
16.12

$
16.35

 
$
17.24

 
868,650

 
9.24
 
$
17.13

 
23,999

 
$
16.85

$
17.36

 
$
22.76

 
1,415,700

 
7.14
 
$
21.18

 
927,334

 
$
21.23

$
22.84

 
$
31.78

 
133,820

 
8.86
 
$
27.47

 
31,312

 
$
23.74
Summary of RSU Activity
The following table summarizes RSU awards granted during the years ended December 31, 2017, 2016 and 2015:
 
Number of
RSUs
 
Weighted
Average
Grant Price
Outstanding at January 1, 2015
20,502

 
$
19.47

Granted
49,776

 
16.07

Released
(26,724
)
 
(18.68
)
Forfeited
—

 
—

Outstanding at December 31, 2015
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
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, 2017, 2016 and 2015:
 
2017
 
2016
 
2015
 
(in millions)
Research and development expenses
$
6.5

 
$
6.2

 
$
4.0

General and administrative expenses
11.6

 
11.8

 
11.6

Total (1)
$
18.1

 
$
18.0

 
$
15.6


_________________________________

(1)
Includes $0.0 million, $1.7 million and $2.3 million for the years ended December 31, 2017, 2016 and 2015, respectively, for the remeasurement of certain stock options and RSUs that occurred during May 2013.