Stock-Based Compensation - Employee expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 6,512	$ 5,856
Employee stock options
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	5,882	5,525
ESPP
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	194	217
Restricted stock units
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	436	114
General and administrative
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	2,291	1,854
Research and development
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 4,221	$ 4,002