Stock-Based Compensation - Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Stock-Based Compensation
Total stock-based compensation expense	$ 1,928	$ 1,526
Research and development
Stock-Based Compensation
Total stock-based compensation expense	662	461
General and administrative
Stock-Based Compensation
Total stock-based compensation expense	$ 1,266	$ 1,065