Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock-based compensation
Total stock-based compensation expense	$ 2,169	$ 2,507
Unrecognized compensation cost	8,315
Stock options
Stock-based compensation
Unrecognized compensation cost	157
RSUs
Stock-based compensation
Unrecognized compensation cost	1,657
RSAs
Stock-based compensation
Unrecognized compensation cost	4,068
Research and development
Stock-based compensation
Total stock-based compensation expense	0	178
General and administrative
Stock-based compensation
Total stock-based compensation expense	2,169	$ 2,329
Market-based | RSUs
Stock-based compensation
Unrecognized compensation cost	728
Market-based | RSAs
Stock-based compensation
Unrecognized compensation cost	$ 1,705