Accounting for Shared-based Payments - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jun. 08, 2015	Jun. 07, 2015
Shared-based payments
Total stock-based compensation expense	$ 12,575	$ 9,724			$ 4,722
Research and development | Employee
Shared-based payments
Total stock-based compensation expense		2,452	$ 3,451		1,914
General and administrative | Employee
Shared-based payments
Total stock-based compensation expense		$ 7,272	$ 9,124		$ 2,808
2008 Equity Compensation Incentive Plan
Shared-based payments
Number of shares reserved under the plan (in shares)						30,000,000	15,000,000
Stock Options
Shared-based payments
Vesting period			3 years
Contractual term			10 years
Unrecognized compensation cost related to non-vested employee stock options, net of expected forfeitures	$ 18,400		$ 18,400
Weighted-average remaining vesting period over which unrecognized compensation cost is to be recognized			1 year 4 months 24 days
Number of stock options which vest upon a change of control (in shares)	2,159,500		2,159,500
Weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
Risk-free interest rate, minimum			1.13%	1.46%	1.78%
Risk-free interest rate, maximum			2.19%	2.02%	2.30%
Dividend yield			0.00%	0.00%	0.00%
Expected volatility, minimum			50.00%	50.00%	52.00%
Expected volatility, maximum			60.00%	80.00%	60.00%
Expected term			6 years	6 years	6 years
Weighted-average fair value per share of options granted (in dollars per share)			$ 1.93	$ 3.79	$ 1.98