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Accounting for Shared-based Payments (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of stock-based compensation expense
Stock-based compensation has been recognized in operating results as follows:
 
 
Year Ended
December 31,
($ in thousands)
 
2017
 
2016
 
2015
Included in research and development
 
$
2,786

 
$
3,451

 
$
2,452

Included in general and administrative
 
19,929

 
9,124

 
7,272

Total stock-based compensation expense
 
$
22,715

 
$
12,575

 
$
9,724
Schedule of weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
The estimated fair value of stock option awards was determined on the date of grant using the Black-Scholes option valuation model with the following assumptions during the periods indicated.
 
Year Ended
December 31,
 
2017
 
2016
 
2015
Risk-free interest rate
1.85%-2.40%

 
1.13%-2.19%

 
1.46%-2.02%

Dividend yield
—

 
—

 
—

Expected volatility
62%-73%

 
50%-60%

 
50%-80%

Expected term (in years)
6 years

 
6 years

 
6 years
Summary of stock option activity and of changes in stock options outstanding under the Plan
A summary of stock option activity and of changes in stock options outstanding under the Plan is presented below:
 
Number of
Options
 
Exercise Price
Per Share
 
Weighted Average
Exercise Price
Per Share
 
Intrinsic
Value
(in thousands)
 
Weighted Average
Remaining
Contractual Term
Balance outstanding, December 31, 2014
16,567,020

 
$0.44-17.79
 
$
3.20

 
$
8,949

 
7.29 years
Granted
4,961,112

 
$2.94-9.33
 
$
6.51

 
$
—

 
 
Exercised
(269,720
)
 
$2.98-6.28
 
$
4.24

 
$
904

 
 
Forfeited
(305,037
)
 
$2.98-9.45
 
$
6.22

 
$
—

 
 
 
 
 
 
 
 
 
 
 
 
Balance outstanding, December 31, 2015
20,953,375

 
$0.44-9.12
 
$
3.86

 
$
42,438

 
7.15 years
Granted
7,537,000

 
$2.93-5.63
 
$
3.99

 
$
—

 

Exercised
(70,185
)
 
$2.94-4.61
 
$
3.17

 
$
143

 

Forfeited
(553,019
)
 
$2.98-9.12
 
$
5.98

 
$
—

 

 
 
 
 
 
 
 
 
 
 
Balance outstanding, December 31, 2016 (1)
27,867,171

 
$0.44-9.12
 
$
3.78

 
$
65,618

 
7.05 years
Granted
3,198,000

 
$1.89-6.77
 
$
3.95

 
$
—

 
 
Exercised
(99,978
)
 
$0.50-5.34
 
$
5.48

 
$
201

 
 
Forfeited
(1,096,902
)
 
$0.50-7.91
 
$
4.65

 
$
—

 
 
Balance outstanding, December 31, 2017 (1)
29,868,291

 
$0.44-9.12
 
$
3.83

 
$
5,346

 
6.09 years
 
 
 
 
 
 
 
 
 
 
Exercisable, at December 31, 2017 (1)
21,576,652

 
$0.44-9.12
 
$
4.52

 
$
5,107

 
5.05 years
__________________________
(1)  Number of options represented above includes 2,159,500 options that were modified to immediately vest during June 2017.