Accounting for Shared-based Payments - Stock Options (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 08, 2015	Jun. 07, 2015
Shared-based payments
Total stock-based compensation expense		$ 13,298	$ 2,707	$ 16,195	$ 3,909
Research and development | Employee
Shared-based payments
Total stock-based compensation expense		(80)	784	1,793	1,594
General and administrative | Employee
Shared-based payments
Total stock-based compensation expense		$ 13,378	$ 1,923	$ 14,402	$ 2,315
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		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	$ 17,700	$ 17,700		$ 17,700
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized				1 year 4 months 21 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%	1.22%
Risk-free interest rate, maximum				2.24%	1.49%
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				66.00%
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%