Stock-Based Compensation - Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee stock options
Weighted-Average Assumptions
Expected term (in years)	0 years
Risk-free interest rate, min (as percent)		1.50%	1.20%
Risk-free interest rate, max (as percent)		2.20%	2.10%
Risk-free interest rate (as percent)	0.00%
Expected volatility, min (as percent)		23.70%	43.00%
Expected volatility, max (as percent)		43.80%	45.30%
Expected volatility (as percent)	0.00%
Employee stock options | Minimum
Weighted-Average Assumptions
Expected term (in years)		2 months 12 days	6 years
Employee stock options | Maximum
Weighted-Average Assumptions
Expected term (in years)		6 years 1 month 6 days	6 years 1 month 6 days
Employee Stock Purchase Plan
Weighted-Average Assumptions
Expected term (in years)	6 months	6 months	0 years
Risk-free interest rate, min (as percent)	1.80%
Risk-free interest rate, max (as percent)	2.40%
Risk-free interest rate (as percent)		1.20%	0.00%
Expected volatility, min (as percent)	22.20%
Expected volatility, max (as percent)	36.40%
Expected volatility (as percent)		28.50%	0.00%