Accounting for Shared-based Payments - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 08, 2015	Jun. 07, 2015
Shared-based payments
Total stock-based compensation expense			$ 22,715	$ 12,575	$ 9,724
Research and development | Employee
Shared-based payments
Total stock-based compensation expense			2,786	3,451	2,452
General and administrative | Employee
Shared-based payments
Total stock-based compensation expense			$ 19,929	$ 9,124	$ 7,272
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
Grants in period (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		2,159,500
Share-based compensation expense		$ 6,800
Unrecognized compensation cost related to non-vested employee stock options, net of expected forfeitures			$ 14,700
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized			1 year 5 months 27 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%	1.46%
Risk-free interest rate, maximum			2.40%	2.19%	2.02%
Dividend yield			0.00%	0.00%	0.00%
Expected volatility, minimum			62.00%	50.00%	50.00%
Expected volatility, maximum			73.00%	60.00%	80.00%
Expected term			6 years	6 years	6 years
Weighted-average fair value per share of options granted (in dollars per share)			$ 2.47	$ 1.93	$ 3.79