Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
BALANCE, Shares at Dec. 31, 2014		163,562,149
BALANCE, Amount at Dec. 31, 2014	$ 2,072.1	$ 1.6	$ 61.8	$ 3,756.6	$ (1,747.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	442.6			442.6
Repurchases of common stock	(237.3)				(237.3)
Treasury stock cancellation, Shares		(43,000,000)
Treasury stock cancellation	0.0	$ (0.4)	(78.7)	(1,496.4)	1,575.5
Stock-based compensation expense	24.0		24.0
Shares awarded under stock-based compensation plans, including excess income tax benefit of $0.6 in 2016 and $17.9 in 2015	47.9		(1.9)		49.8
BALANCE, Shares at Dec. 31, 2015		120,562,149
BALANCE, Amount at Dec. 31, 2015	2,349.3	$ 1.2	5.2	2,702.8	(359.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	430.5			430.5
Repurchases of common stock	(499.0)				(499.0)
Stock-based compensation expense	25.1		25.1
Shares awarded under stock-based compensation plans, including excess income tax benefit of $0.6 in 2016 and $17.9 in 2015	9.0		(7.5)		16.5
Other	(4.6)		(4.6)
BALANCE, Shares at Dec. 31, 2016		120,562,149
BALANCE, Amount at Dec. 31, 2016	2,310.3	$ 1.2	18.2	3,133.3	(842.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	434.6			434.6
Repurchases of common stock	(436.0)				(436.0)
Treasury stock cancellation, Shares		(18,000,000)
Treasury stock cancellation	0.0	$ (0.2)	(30.2)	(735.6)	766.0
Stock-based compensation expense	20.6		20.6
Shares awarded under stock-based compensation plans, including excess income tax benefit of $0.6 in 2016 and $17.9 in 2015	39.7		(4.8)		44.5
Other	0.1		0.2	(0.1)
BALANCE, Shares at Dec. 31, 2017		102,562,149
BALANCE, Amount at Dec. 31, 2017	$ 2,369.3	$ 1.0	$ 4.0	$ 2,832.2	$ (467.9)