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Accounting for Shared-Based Payments - Stock Options (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Jun. 08, 2015
Jun. 07, 2015
Shared-based payments            
Total stock-based compensation expense (income) $ 2,707 $ 5,389 $ 3,909 $ 7,794    
Research and development | Employee            
Shared-based payments            
Total stock-based compensation expense (income) 784 580 1,594 1,110    
General and administrative | Employee            
Shared-based payments            
Total stock-based compensation expense (income) 1,923 $ 4,809 $ 2,315 $ 6,684    
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 $ 22,000   $ 22,000      
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized     2 years 7 months 28 days      
Number of stock options which vest upon a change of control (in shares) 4,319,000   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)     1.22% 1.46%    
Risk-free interest rate, maximum (as a percent)     1.49% 2.35%    
Dividend yield (as percent)     0.00% 0.00%    
Expected volatility (as a percent)     50.00% 60.00%    
Expected term     6 years 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