Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 385.9		$ 378.7		$ 370.6		$ 353.8		$ 342.8		$ 332.0		$ 315.4		$ 295.6		$ 1,489.0	$ 1,285.8	$ 1,110.0
Income before provision for income taxes	215.2		219.1		215.3		206.3		201.0		198.4		198.0		157.7		855.9	755.1	583.8
Net income	$ 161.9		$ 165.4		$ 164.4		$ 164.4		$ 151.9		$ 151.0		$ 151.0		$ 120.1		$ 656.1	$ 574.0	$ 470.2
Net income per share: Basic	$ 8.63	[1]	$ 8.73	[1]	$ 8.68	[1]	$ 8.67	[1]	$ 7.82	[1]	$ 7.76	[1]	$ 7.76	[1]	$ 6.18	[1]	$ 34.71	$ 29.52	$ 24.12
Net income per share: Diluted	$ 8.60	[1]	$ 8.73	[1]	$ 8.68	[1]	$ 8.65	[1]	$ 7.79	[1]	$ 7.75	[1]	$ 7.75	[1]	$ 6.17	[1]	$ 34.57	$ 29.39	$ 24.04

[1] Basic and diluted net income per share are computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share information may not equal annual basic and diluted net income per share.