Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-based compensation
Total stock-based compensation expense	$ 1,452	$ 2,427	$ 3,621	$ 4,934
Unrecognized compensation cost	7,756		7,756
Stock options
Stock-based compensation
Unrecognized compensation cost	19		19
RSUs
Stock-based compensation
Unrecognized compensation cost	2,096		2,096
RSAs
Stock-based compensation
Unrecognized compensation cost	3,521		3,521
Research and development
Stock-based compensation
Total stock-based compensation expense	0	175	0	353
General and administrative
Stock-based compensation
Total stock-based compensation expense	1,452	$ 2,252	3,621	$ 4,581
Market-based | RSUs
Stock-based compensation
Unrecognized compensation cost	627		627
Market-based | RSAs
Stock-based compensation
Unrecognized compensation cost	$ 1,493		$ 1,493