Stock-Based Compensation - Employee expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 4,882	$ 3,448	$ 9,353	$ 6,607
Employee stock options
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	4,614	3,319	8,891	6,358
ESPP
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	190	129	353	249
Restricted stock units
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	78		109
General and administrative
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	1,688	1,416	3,305	2,884
Research and development
Stock-based compensation
Total employee, director and non-employee stock-based compensation expense	$ 3,194	$ 2,032	$ 6,048	$ 3,723