Stock-Based Compensation - Assumptions (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Employee stock options
Weighted-Average Assumptions
Expected term (in years)	0 days	6 years 1 month 6 days	0 days
Risk-free interest rate, min (as percent)		1.90%		1.90%
Risk-free interest rate, max (as percent)		2.10%		2.10%
Risk-free interest rate (as percent)	0.00%		0.00%
Expected volatility, min (as percent)		38.90%		38.90%
Expected volatility, max (as percent)		39.10%		43.80%
Expected volatility (as percent)	0.00%		0.00%
Employee stock options | Minimum
Weighted-Average Assumptions
Expected term (in years)				6 years
Employee stock options | Maximum
Weighted-Average Assumptions
Expected term (in years)				6 years 1 month 6 days
Employee Stock Purchase Plan
Weighted-Average Assumptions
Expected term (in years)	6 months	6 months	6 months	6 months
Risk-free interest rate, min (as percent)			1.80%
Risk-free interest rate, max (as percent)			2.40%
Risk-free interest rate (as percent)	2.40%	1.20%		1.20%
Expected volatility, min (as percent)			22.20%
Expected volatility, max (as percent)			36.40%
Expected volatility (as percent)	36.40%	28.50%		28.50%