Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 342.8		$ 332.0		$ 315.4		$ 295.6		$ 287.3		$ 283.9		$ 276.0		$ 262.8		$ 1,285.8	$ 1,110.0	$ 969.2
Income before provision for income taxes	201.0		198.4		198.0		157.7		122.4		160.1		157.1		144.2		755.1	583.8	531.2
Net income	$ 151.9		$ 151.0		$ 151.0		$ 120.1		$ 177.1		$ 100.7		$ 99.1		$ 93.3		$ 574.0	$ 470.2	$ 332.8
Net income per share: Basic	$ 7.82	[1]	$ 7.76	[1]	$ 7.76	[1]	$ 6.18	[1]	$ 9.13	[1]	$ 5.19	[1]	$ 5.09	[1]	$ 4.73	[1]	$ 29.52	$ 24.12	$ 16.37
Net income per share: Diluted	$ 7.79	[1]	$ 7.75	[1]	$ 7.75	[1]	$ 6.17	[1]	$ 9.10	[1]	$ 5.19	[1]	$ 5.09	[1]	$ 4.72	[1]	$ 29.39	$ 24.04	$ 16.31

[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.