Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 287.3		$ 283.9		$ 276.0		$ 262.8		$ 256.2		$ 246.6		$ 238.5		$ 227.9		$ 1,110.0	$ 969.2	$ 825.3
Income before provision for income taxes	122.4		160.1		157.1		144.2		141.6		136.5		134.7		118.4		583.8	531.2	474.5
Net income	$ 177.1		$ 100.7		$ 99.1		$ 93.3		$ 87.6		$ 85.9		$ 84.9		$ 74.4		$ 470.2	$ 332.8	$ 299.7
Net income per share: Basic	$ 9.13	[1]	$ 5.19	[1]	$ 5.09	[1]	$ 4.73	[1]	$ 4.35	[1]	$ 4.22	[1]	$ 4.17	[1]	$ 3.64	[1]	$ 24.12	$ 16.37	$ 14.35
Net income per share: Diluted	$ 9.10	[1]	$ 5.19	[1]	$ 5.09	[1]	$ 4.72	[1]	$ 4.33	[1]	$ 4.21	[1]	$ 4.17	[1]	$ 3.63	[1]	$ 24.04	$ 16.31	$ 14.28

[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.