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Accounting for Shared-based Payments (Tables)
 9 Months Ended
Sep. 30, 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:
 
 
Three Months Ended
September 30,
 
Three Months Ended
September 30,
 
Nine Months Ended
September 30,
 
Nine Months Ended
September 30,
($ in thousands)
 
2017
 
2016
 
2017
 
2016
Included in research and development
 
$
477

 
$
961

 
$
2,270

 
$
2,555

Included in selling, general and administrative
 
3,411

 
3,624

 
17,812

 
5,939

Total stock-based compensation expense
 
$
3,888

 
$
4,585

 
$
20,082

 
$
8,494
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 weighted-average assumptions during the periods indicated.
 
Nine Months Ended
September 30, 2017
 
Nine Months Ended
September 30, 2016
Risk-free interest rate
1.85%-2.24%

 
1.13%-1.74%

Dividend yield
—

 
—

Expected volatility
62%-73%

 
50
%
Expected term (in 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, 2016
27,867,171

 
$0.44-9.12
 
$
3.78

 
$
65,618

 
7.1 years
Granted
2,133,000

 
$3.79
 
4.93

 
—

 

Exercised
(99,978
)
 
$0.50-5.34
 
5.48

 
201

 

Forfeited
(1,089,402
)
 
$2.61-7.91
 
4.64

 
—

 

Balance outstanding, September 30, 2017
28,810,791

 
$0.44-9.12
 
$
3.89

 
$
7,936

 
6.2 years
 
 
 
 
 
 
 
 
 
 
Exercisable, at September 30, 2017 (1)
20,041,157

 
$0.44-9.12
 
$
4.36

 
$
7,936

 
5.1 years
__________________________

(1)
Includes 2,159,500 change of control stock options that were modified to immediately vest during June 2017.