Unaudited Condensed Consolidated Statement Of Shareholders' Equity - 9 months ended Sep. 30, 2017 - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
BALANCE at period start (in shares) at Dec. 31, 2016		120,562,149
BALANCE at period start 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	283.3			283.3
Repurchases of common stock	(436.0)				(436.0)
Stock-based compensation expense	18.1		18.1
Shares awarded under stock-based compensation plans	24.7		(6.3)		31.0
Other	0.1		0.2	(0.1)
BALANCE at period end (in shares) at Sep. 30, 2017		120,562,149
BALANCE at period end at Sep. 30, 2017	$ 2,200.5	$ 1.2	$ 30.2	$ 3,416.5	$ (1,247.4)