Stockholders' Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Stock-Based Compensation Expense
Total stock-based compensation expense	$ 839	$ 479	$ 1,460	$ 969
Research and development
Stock-Based Compensation Expense
Total stock-based compensation expense	139	159	337	310
General and administrative expenses
Stock-Based Compensation Expense
Total stock-based compensation expense	$ 700	$ 320	$ 1,123	$ 659