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Accounting for Shared-based Payments - Stock Options (Details) - USD ($)
3 Months Ended
Nov. 20, 2009
Mar. 31, 2017
Mar. 31, 2016
Jun. 08, 2015
Jun. 07, 2015
Shared-based payments          
Total stock-based compensation expense   $ 2,897,000 $ 1,202,000    
Research and development | Employee          
Shared-based payments          
Total stock-based compensation expense   1,873,000 810,000    
General and administrative | Employee          
Shared-based payments          
Total stock-based compensation expense   $ 1,024,000 $ 392,000    
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   $ 16,400,000      
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized   2 years 1 month 28 days      
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model          
Risk-free interest rate, minimum   1.96% 1.47%    
Risk-free interest rate, maximum   2.26% 1.74%    
Dividend yield   0.00% 0.00%    
Expected volatility   50.00% 60.00%    
Expected term   6 years 6 years    
Contingent vesting | $0.44-9.12 | Stock Options          
Shared-based payments          
Total stock-based compensation expense $ 0        
Number of stock options which vest upon a change of control (in shares)   2,159,500