Statements of Comprehensive Loss Parentheticals (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based compensation expense associated with research and development expense	$ 7,096	$ 6,010	$ 4,905
Stock-based compensation expense associated with general and administrative expense	$ 7,122	$ 5,686	$ 4,567