Stock-Based Compensation - FV of employee stock options (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility (as a percent)	53.90%	61.30%
Risk-free interest rate (as a percent)	1.71%	2.53%
Expected term (years)	6 years 3 months 18 days	6 years 1 month 6 days
Compensation expense
Unamortized compensation expense related to unvested options	$ 62.2
Period to recognize unamortized compensation expense	2 years 10 months 24 days
ESPP
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)		57.00%
Expected volatility, high end of range (as a percent)		71.40%
Expected volatility (as a percent)	50.80%
Risk-free interest rate, low end of range (as a percent)	1.56%	1.47%
Risk-free interest rate, high end of range (as a percent)	1.65%	2.70%
Compensation expense
Unamortized compensation expense related to unvested options	$ 1.3
Period to recognize unamortized compensation expense	1 year 8 months 12 days
ESPP | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	6 months	6 months
ESPP | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)	2 years	2 years