Stock-Based Compensation - Other (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Stock-based compensation expense
Total stock-based compensation expense	$ 3,765	$ 2,631	$ 6,649	$ 5,184
Research and development | Employee Stock Options
Stock-based compensation expense
Total stock-based compensation expense	849	691	1,624	1,276
Research and development | Non-employees stock options
Stock-based compensation expense
Total stock-based compensation expense	46	40	92	122
General and administrative | Employee Stock Options
Stock-based compensation expense
Total stock-based compensation expense	2,870	1,877	4,921	3,710
General and administrative | Non-employees stock options
Stock-based compensation expense
Total stock-based compensation expense	$ 0	$ 23	$ 12	$ 76