Stock-Based Compensation - Expenses (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-based compensation expense
Stock-based compensation expense	$ 1,464	$ 2,048	$ 3,157	$ 4,536
Stock options
Stock-based compensation expense
Weighted average grant-date fair value (in dollars per share)	$ 2.09		$ 2.09
Research and development
Stock-based compensation expense
Stock-based compensation expense	$ 654	978	$ 1,410	2,289
General and administrative
Stock-based compensation expense
Stock-based compensation expense	$ 810	$ 1,070	$ 1,747	$ 2,247