Condensed Consolidated Statement of Shareholders' Equity (Unaudited) - USD ($) $ in Millions		Total	Common Stock	Additional Paid-In Capital	Deferred Compensation Equity	Accumulated Other Comprehensive Income (loss).	Retained Earnings	Shares in Treasury
Balance at the beginning of the period at Dec. 31, 2015		$ 2,347.4	$ 78.7	$ 293.4	$ 8.0	$ (87.1)	$ 2,381.8	$ (327.4)
Balance (in shares) at Dec. 31, 2015			64,051,600					14,383,241
Increase (Decrease) in Shareholders' Equity
Net income		173.9					173.9
Other comprehensive income (loss), net of tax		(4.6)				(4.6)
Cash dividends - $0.60 and $0.70 for the six months ended June 30, 2016 and 2017, respectively		(61.8)					(61.8)
Repurchases of common stock		(61.9)						$ (61.9)
Repurchases of common stock (in shares)			(656,057)					656,057
Issuances for stock based compensation (1)	[1]	53.9		33.1	2.3			$ 18.5
Issuances for stock based compensation (in shares)	[1]		910,663					(909,521)
Balance at the end of the period at Sep. 30, 2016		2,446.9	$ 78.7	326.5	10.3	(91.7)	2,493.9	$ (370.8)
Balance (in shares) at Sep. 30, 2016			64,306,206					14,129,777
Balance at the beginning of the period at Jun. 30, 2016						(91.4)
Increase (Decrease) in Shareholders' Equity
Net income		(9.8)
Other comprehensive income (loss), net of tax		(0.3)				(0.3)
Cash dividends - $0.60 and $0.70 for the six months ended June 30, 2016 and 2017, respectively		(22.8)
Balance at the end of the period at Sep. 30, 2016		2,446.9	$ 78.7	326.5	10.3	(91.7)	2,493.9	$ (370.8)
Balance (in shares) at Sep. 30, 2016			64,306,206					14,129,777
Balance at the beginning of the period at Dec. 31, 2016		$ 2,466.9	$ 78.7	335.3	10.3	(122.2)	2,547.4	$ (382.6)
Balance (in shares) at Dec. 31, 2016		64,257,182	64,257,182					14,178,801
Increase (Decrease) in Shareholders' Equity
Net income		$ 250.4					250.4
Other comprehensive income (loss), net of tax		44.9				44.9
Cash dividends - $0.60 and $0.70 for the six months ended June 30, 2016 and 2017, respectively		(69.0)					(69.0)
Repurchases of common stock		(268.5)						$ (268.5)
Repurchases of common stock (in shares)			(2,719,538)					2,719,538
Issuances for stock based compensation (1)	[1]	11.6		11.5	1.6			$ (1.5)
Issuances for stock based compensation (in shares)	[1]		225,434					(183,754)
Balance at the end of the period at Sep. 30, 2017		$ 2,436.3	$ 78.7	346.8	11.9	(77.3)	2,728.8	$ (652.6)
Balance (in shares) at Sep. 30, 2017		61,763,078	61,763,078					16,714,585
Balance at the beginning of the period at Jun. 30, 2017						(90.9)
Increase (Decrease) in Shareholders' Equity
Net income		$ 86.3
Other comprehensive income (loss), net of tax		13.6				13.6
Cash dividends - $0.60 and $0.70 for the six months ended June 30, 2016 and 2017, respectively		(23.2)
Balance at the end of the period at Sep. 30, 2017		$ 2,436.3	$ 78.7	$ 346.8	$ 11.9	$ (77.3)	$ 2,728.8	$ (652.6)
Balance (in shares) at Sep. 30, 2017		61,763,078	61,763,078					16,714,585

[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.