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Accounting for Shared-Based Payments - Stock Options (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Jun. 08, 2015
Jun. 07, 2015
Shared-based payments        
Total stock-based compensation expense (income) $ 1,202 $ 2,405    
Research and development | Employee        
Shared-based payments        
Total stock-based compensation expense (income) 810 530    
General and administrative | Employee        
Shared-based payments        
Total stock-based compensation expense (income) $ 392 $ 1,875    
2008 Equity Compensation Incentive Plan        
Shared-based payments        
Number of shares reserved under the plan (in shares)     30,000,000 15,000,000
Stock Options        
Shared-based payments        
Vesting period 3 years      
Contractual term 10 years      
Unrecognized compensation cost related to non-vested employee stock options, net of expected forfeitures $ 16,300      
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized 1 year 11 months 9 days      
Number of stock options which vest upon a change of control (in shares) 4,319,000      
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model        
Risk-free interest rate, minimum (as a percent) 1.47% 1.46%    
Risk-free interest rate, maximum (as a percent) 1.74% 2.02%    
Dividend yield (as percent) 0.00% 0.00%    
Expected volatility (as a percent) 60.00% 57.00%    
Expected term 6 years      
Minimum | Stock Options        
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model        
Expected term   6 years    
Maximum | Stock Options        
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model        
Expected term   9 years 3 months 18 days