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Accounting for Shared-based Payments (Tables)
 9 Months Ended
Sep. 30, 2018
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) 2018201720182017
Included in research and development $459 $477 $1,634 $2,270 
Included in selling, general and administrative 2,525 3,411 7,737 17,812 
Total stock-based compensation expense(1)
$2,984 $3,888 $9,371 $20,082 
_____________________
(1) Stock based compensation expense for the nine months ended September 30, 2018 includes $193,000 recorded as a liability.
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, 2018 		Nine Months Ended
September 30, 2017 
Risk-free interest rate 2.33-2.85% 1.85%-2.24% 
Dividend yield — — 
Expected volatility 63%-68% 62%-73% 
Expected term 6 years6 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 Plans 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, 2017 29,868,291 $0.44-9.12 $3.83 $5,346 6.09 years
Granted 7,917,700 $1.56-2.40 $2.12 $— 
Exercised (1)
(1,805,081)$0.50-1.20 $0.56 $2,153 
Forfeited (2,680,222)$0.50-7.91 $2.78 $— 
Balance outstanding, September 30, 2018 33,300,688 $0.44-9.12 $3.61 $714 6.60 years
Exercisable, at September 30, 2018 22,056,762 $0.44-9.12 $4.00 $692 5.41 years
____________________
(1) Options exercised includes 428,147 shares withheld for payment of exercise price and related taxes.