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Shareholders’ Equity (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity and Related Data
The Company’s 2013 Equity Plan stock option activity for the years ended December 31, 2017, 2016 and 2015 was comprised of the following:
 
Outstanding Stock Options and 2013 Equity Plans
 
Shares
Underlying
Stock Options
 
Weighted-
Average
Exercise Price
per Share
 
Weighted-
Average
Remaining
Contractual
Term
 
Aggregate
Intrinsic
Value
Outstanding at December 31, 2014
618,900

 
$
9.54

 
 
 
 
Granted
1,769,785

 
$
25.89

 
 
 
 
Exercised
(51,814
)
 
$
10.81

 
 
 
 
Forfeited
(18,186
)
 
$
7.80

 
 
 
 
Outstanding at December 31, 2015
2,318,685

 
$
22.01

 
 
 
 
Granted
483,200

 
$
17.83

 
 
 
 
Exercised
(17,548
)
 
$
8.52

 
 
 
 
Forfeited
(156,875
)
 
$
26.35

 
 
 
 
Outstanding at December 31, 2016
2,627,462

 
$
21.07

 
 
 
 
Granted
3,704,725

 
$
25.94

 
 
 
 
Exercised
(174,628
)
 
$
15.42

 
 
 
 
Forfeited
(120,257
)
 
$
22.82

 
 
 
 
Outstanding at December 31, 2017
6,037,302

 
$
24.19

 
8.76 years
 
$
49,399,882

Vested and expected to vest at December 31, 2017
6,037,302

 
$
24.19

 
8.76 years
 
$
49,399,882

Exercisable at December 31, 2017
1,536,369

 
$
20.94

 
7.36 years
 
$
17,799,721
Summary of restricted stock activity
The Company’s RSA activity for the years ended December 31, 2017, 2016 and 2015 was comprised of the following:
 
Number of
Shares
 
Weighted-
Average Grant
Date Fair
Market Value
Unvested at December 31, 2014
1,279,007

 
$
12.86

Granted
4,000

 
$
23.12

Vested
(210,108
)
 
$
12.41

Unvested at December 31, 2015
1,072,899

 
$
13.00

Vested
(530,219
)
 
$
12.78

Unvested at December 31, 2016
542,680

 
$
13.22

Vested
(542,680
)
 
$
13.22

Unvested at December 31, 2017
—

 
—
Stock Options, Valuation Assumptions
The Company estimated the fair value of each stock option grant on the grant date using the Black-Scholes model with the following weighted-average assumptions:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Volatility
139
%
 
143
%
 
149
%
Expected life (years)
6.12

 
5.80

 
5.28

Risk-free interest rate
2.1
%
 
1.4
%
 
1.5
%
Dividend yield
—

 
—

 
—
Summary of Share-based Compensation Expense
Total share-based compensation expense related to all share-based awards for the years ended December 31, 2017, 2016 and 2015 was comprised of the following (in thousands):
 
Year Ended December 31,
 
2017
 
2016
 
2015
Research and development:
 
 
 
 
 
   Stock options
$
11,904

 
$
5,599

 
$
2,428

   Restricted stock
—

 
30

 
1,600

   Warrants
76

 
28

 
56

Research and development share-based compensation expense
11,980


5,657


4,084

General and administrative:
 
 
 
 
 
   Stock options
8,837

 
6,539

 
3,693

   Restricted stock
409

 
2,161

 
4,265

   Warrants
569

 
189

 
1,030

General and administrative share-based compensation expense
9,815


8,889


8,988

Total share-based compensation expense included in expenses
$
21,795


$
14,546


$
13,072