Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility (as a percent)	72.90%	88.90%	73.10%	89.20%
Risk-free interest rate (as a percent)	2.77%	1.91%	2.53%	2.04%
Expected term (years)	5 years 10 months 24 days	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Compensation expense
Unamortized compensation expense related to unvested options	$ 47.9		$ 47.9
Period to recognize unamortized compensation expense			3 years
ESPP
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)	61.20%	67.80%	61.20%	67.80%
Expected volatility, high end of range (as a percent)	71.40%	79.80%	71.40%	79.80%
Risk-free interest rate, low end of range (as a percent)	1.47%	0.47%	1.47%	0.47%
Risk-free interest rate, high end of range (as a percent)	2.41%	1.09%	2.41%	1.09%
Compensation expense
Unamortized compensation expense related to unvested options	$ 1.2		$ 1.2
Period to recognize unamortized compensation expense			1 year 4 months 24 days
ESPP | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months	6 months	6 months
ESPP | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years	2 years	2 years