Condensed Consolidated Statement of Shareholders' Equity (Unaudited) - USD ($) $ in Millions		Total	Common Stock		Additional Paid-In Capital	Accumulated Other Comprehensive Income (loss)	Retained Earnings	Shares in Treasury
Balance at the beginning of the period at Dec. 31, 2019		$ 2,642.8	$ 78.7		$ 416.6	$ (124.1)	$ 3,721.3	$ (1,449.7)
Balance (in shares) at Dec. 31, 2019			55,700,000					22,700,000
Increase (Decrease) in Shareholders' Equity
Net income		239.5					239.5
Other comprehensive income (loss), net of tax		(7.8)				(7.8)
Dividends		(84.7)					(84.7)
Repurchases of common stock		(341.7)						$ (341.7)
Repurchases of common stock (in shares)			(2,700,000)					(2,700,000)
Issuances and deferrals. net for stock based compensation	[1]	27.2			17.0			$ 10.2
Issuances and deferrals, net for stock based compensation (in shares)	[1]		200,000					(200,000)
Balance at the end of the period at Sep. 30, 2020		2,475.3	$ 78.7		433.6	(131.9)	3,876.1	$ (1,781.2)
Balance (in shares) at Sep. 30, 2020			53,200,000					25,200,000
Balance at the beginning of the period at Jun. 30, 2020		2,516.2	$ 78.7		427.8	(160.1)	3,802.5	$ (1,632.7)
Balance (in shares) at Jun. 30, 2020			54,400,000					24,000,000.0
Increase (Decrease) in Shareholders' Equity
Net income		102.3					102.3
Other comprehensive income (loss), net of tax		28.2				28.2
Dividends		(28.7)					(28.7)
Repurchases of common stock		(149.9)						$ (149.9)
Repurchases of common stock (in shares)			(1,200,000)					(1,200,000)
Issuances and deferrals. net for stock based compensation	[1]	7.2			5.8			$ 1.4
Issuances and deferrals, net for stock based compensation (in shares)	[1]							0
Balance at the end of the period at Sep. 30, 2020		2,475.3	$ 78.7		433.6	(131.9)	3,876.1	$ (1,781.2)
Balance (in shares) at Sep. 30, 2020			53,200,000					25,200,000
Balance at the beginning of the period at Dec. 31, 2020		$ 2,537.7	$ 78.7		441.7	(97.0)	3,928.7	$ (1,814.4)
Balance (in shares) at Dec. 31, 2020		52,900,000	52,900,000					25,500,000
Increase (Decrease) in Shareholders' Equity
Net income		$ 293.6					293.6
Other comprehensive income (loss), net of tax		(4.2)				(4.2)
Dividends		(84.4)					(84.4)
Repurchases of common stock		(290.6)						$ (290.6)
Repurchases of common stock (in shares)			(1,700,000)					(1,700,000)
Issuances and deferrals. net for stock based compensation	[1]	92.7			32.1			$ 60.6
Issuances and deferrals, net for stock based compensation (in shares)	[1]		900,000					(900,000)
Balance at the end of the period at Sep. 30, 2021		$ 2,544.8	$ 78.7		473.8	(101.2)	4,137.9	$ (2,044.4)
Balance (in shares) at Sep. 30, 2021		52,100,000	52,100,000					26,300,000
Balance at the beginning of the period at Jun. 30, 2021		$ 2,417.9	$ 78.7		460.1	(102.8)	4,024.2	$ (2,042.3)
Balance (in shares) at Jun. 30, 2021			51,900,000					26,500,000
Increase (Decrease) in Shareholders' Equity
Net income		142.1					142.1
Other comprehensive income (loss), net of tax		1.6				1.6
Dividends		(28.4)					(28.4)
Repurchases of common stock		(25.0)						$ (25.0)
Repurchases of common stock (in shares)			(100,000)					(100,000)
Issuances and deferrals. net for stock based compensation	[1]	36.6			13.7			$ 22.9
Issuances and deferrals, net for stock based compensation (in shares)			300,000	[1]				(300,000)
Balance at the end of the period at Sep. 30, 2021		$ 2,544.8	$ 78.7		$ 473.8	$ (101.2)	$ 4,137.9	$ (2,044.4)
Balance (in shares) at Sep. 30, 2021		52,100,000	52,100,000					26,300,000

[1] Issuances and deferrals, net for stock-based compensation reflects share activity related to option exercises, restricted and performance shares vested, and net issuances and deferrals associated with deferred compensation equity.