Stock Based Compensation - Total Stock-based Compensation Recorded Related to Options Granted to Employees and Nonemployees and Employee Stock Purchase Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-based compensation expense:
Stock-based compensation expense	$ 10,095	$ 3,986	$ 553
Research and development
Stock-based compensation expense:
Stock-based compensation expense	4,925	1,972	195
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 5,170	$ 2,014	$ 358