Stock-Based Compensation - FV of employee stock options (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility (as a percent)	88.90%	76.10%	89.20%	75.80%
Risk-free interest rate (as a percent)	1.91%	1.36%	2.04%	1.53%
Expected term (years)	6 years 1 month 6 days	5 years 10 months 24 days	6 years 1 month 6 days	6 years 1 month 6 days
Unamortized compensation expense related to unvested options	$ 35,400,000	$ 16,300,000	$ 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%	67.80%	67.80%	67.80%
Expected volatility, high end of range (as a percent)	79.80%	79.60%	79.80%	79.60%
Risk-free interest rate, low end of range (as a percent)	0.47%	0.47%	0.47%	0.47%
Risk-free interest rate, high end of range (as a percent)	1.09%	0.93%	1.09%	0.93%
Unamortized compensation expense related to unvested options	$ 242,305	$ 491,000	$ 242,305	$ 491,000
Period to recognize unamortized compensation expense			5 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	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	2 years	2 years