Stock-Based Compensation - FV of employee stock options (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Compensation expense
Total employee, director and non-employee stock-based compensation expense	$ 3,158,000	$ 1,960,000
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility (as a percent)	89.20%	75.80%
Risk-free interest rate (as a percent)	2.07%	1.56%
Expected term (years)	6 years 2 months 12 days	6 years 1 month 6 days
Compensation expense
Unamortized compensation expense related to unvested options	$ 35,400,000	$ 16,300,000
Period to recognize unamortized compensation expense	3 years
Employee Stock Purchase Plan
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)	67.80%
Expected volatility, high end of range (as a percent)	79.80%
Expected volatility (as a percent)		67.80%
Risk-free interest rate, low end of range (as a percent)	0.55%	0.55%
Risk-free interest rate, high end of range (as a percent)	0.93%	0.93%
Compensation expense
Unamortized compensation expense related to unvested options	$ 339,000	$ 418,000
Period to recognize unamortized compensation expense	8 months 15 days
Employee Stock Purchase Plan | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months
Employee Stock Purchase Plan | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years
General and administrative
Compensation expense
Total employee, director and non-employee stock-based compensation expense	$ 1,467,000	$ 952,000
Research and development
Compensation expense
Total employee, director and non-employee stock-based compensation expense	$ 1,691,000	$ 1,008,000