Statements of Comprehensive Loss Parentheticals (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense associated with research and development expense	$ 4,905	$ 3,938	$ 3,693
Stock-based compensation expense associated with general and administrative expense	$ 4,567	$ 3,514	$ 3,150