Quarterly report pursuant to Section 13 or 15(d)

Accounting for Shared-based Payments - Stock Options (Details)

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Accounting for Shared-based Payments - Stock Options (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended
Jun. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2008
Sep. 30, 2018
Sep. 30, 2017
Jun. 08, 2015
Jun. 07, 2015
Shared-based payments                
Total stock-based compensation expense   $ 2,984 $ 3,888   $ 9,371 $ 20,082    
Stock based compensation liability   193     193      
Research and development | Employee                
Shared-based payments                
Total stock-based compensation expense   459 477   1,634 2,270    
General and administrative | Employee                
Shared-based payments                
Total stock-based compensation expense   2,525 $ 3,411   $ 7,737 $ 17,812    
2008 Equity Compensation Incentive Plan                
Shared-based payments                
Number of shares reserved under the plan (in shares)             30,000,000 15,000,000
2017 Equity Incentive Plan                
Shared-based payments                
Number of shares reserved under the plan (in shares)     9,000,000     9,000,000    
Stock Options                
Shared-based payments                
Vesting period       3 years        
Contractual term       10 years        
Granted (in shares)         7,917,700      
Number of stock options which vest upon a change of control (in shares) 2,159,500              
Share-based compensation expense     $ 6,800          
Unrecognized compensation cost related to non-vested employee stock options, net of expected forfeitures   $ 12,400     $ 12,400      
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized         11 months 19 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         2.33% 1.85%    
Risk-free interest rate, maximum         2.85% 2.24%    
Dividend yield         0.00% 0.00%    
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 volatility         63.00% 62.00%    
Maximum | Stock Options                
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model                
Expected volatility         68.00% 73.00%