Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 447.4		$ 426.5		$ 406.3		$ 389.1		$ 385.9		$ 378.7		$ 370.6		$ 353.8		$ 1,669.3	$ 1,489.0	$ 1,285.8
Income before provision for income taxes	216.1		318.4		127.8		(112.8)		215.2		219.1		215.3		206.3		549.5	855.9	755.1
Net income	$ 166.3		$ 242.1		$ 96.4		$ (83.8)		$ 161.9		$ 165.4		$ 164.4		$ 164.4		$ 421.0	$ 656.1	$ 574.0
Net income per share: Basic	$ 9.47	[1]	$ 13.57	[1]	$ 5.40	[1]	$ (4.61)	[1]	$ 8.63	[1]	$ 8.73	[1]	$ 8.68	[1]	$ 8.67	[1]	$ 23.57	$ 34.71	$ 29.52
Net income per share: Diluted	$ 9.43	[1]	$ 13.56	[1]	$ 5.40	[1]	$ (4.61)	[1]	$ 8.60	[1]	$ 8.73	[1]	$ 8.68	[1]	$ 8.65	[1]	$ 23.47	$ 34.57	$ 29.39

[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.