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, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 08, 2015	Jun. 07, 2015
Shared-based payments
Total stock-based compensation expense		$ 3,888		$ 4,585	$ 20,082	$ 8,494
Research and development | Employee
Shared-based payments
Total stock-based compensation expense		477		961	2,270	2,555
General and administrative | Employee
Shared-based payments
Total stock-based compensation expense		$ 3,411		$ 3,624	$ 17,812	$ 5,939
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
Granted (in shares)		0
Stock Options
Shared-based payments
Vesting period					3 years
Contractual term					10 years
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		$ 16,700			$ 16,700
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized					1 year 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					1.85%	1.13%
Risk-free interest rate, maximum					2.24%	1.74%
Dividend yield					0.00%	0.00%
Expected volatility						50.00%
Expected term					6 years	6 years
Stock Options | Minimum
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
Expected volatility					62.00%
Stock Options | Maximum
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
Expected volatility					73.00%