Stockholders' Equity - 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 Expense
Total stock-based compensation expense	$ 987	$ 925	$ 2,106	$ 1,760
Research and development
Stock-Based Compensation Expense
Total stock-based compensation expense	338	303	689	580
General and administrative
Stock-Based Compensation Expense
Total stock-based compensation expense	$ 649	$ 622	$ 1,417	$ 1,180