Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 217.8		$ 210.2		$ 203.1		$ 194.2		$ 185.1		$ 181.7		$ 179.8		$ 176.9		$ 825.3	$ 723.5	$ 682.1
Income before provision for income taxes	125.9		117.9		117.4		113.3		116.4		115.3		109.2		80.0		474.5	420.9	398.2
Net income	$ 80.0		$ 74.0		$ 74.2		$ 71.5		$ 73.0		$ 74.0		$ 69.4		$ 49.8		$ 299.7	$ 266.2	$ 253.1
Net income per share: Basic	$ 3.86	[1]	$ 3.53	[1]	$ 3.54	[1]	$ 3.42	[1]	$ 3.46	[1]	$ 3.38	[1]	$ 3.06	[1]	$ 2.12	[1]	$ 14.35	$ 11.96	$ 10.61
Net income per share: Diluted	$ 3.84	[1]	$ 3.53	[1]	$ 3.54	[1]	$ 3.41	[1]	$ 3.45	[1]	$ 3.38	[1]	$ 3.06	[1]	$ 2.12	[1]	$ 14.28	$ 11.92	$ 10.54

[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.