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, 2018	Jun. 30, 2017	Sep. 30, 2008	Jun. 30, 2018	Jun. 30, 2017	Jun. 08, 2015	Jun. 07, 2015
Shared-based payments
Total stock-based compensation expense		$ 3,428	$ 13,298		$ 6,387	$ 16,195
Stock based compensation liability		193			193
Research and development | Employee
Shared-based payments
Total stock-based compensation expense		504	(80)		1,175	1,793
General and administrative | Employee
Shared-based payments
Total stock-based compensation expense		2,924	$ 13,378		$ 5,212	$ 14,402
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
Stock Options
Shared-based payments
Vesting period				3 years
Contractual term				10 years
Granted (in shares)					7,694,200
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		$ 15,800			$ 15,800
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized					1 year 25 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 volatility min					63.00%	66.00%
Expected volatility max					66.00%	73.00%
Expected term					6 years	6 years