Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share [Abstract]
Net income	$ 134.0	$ 109.5
Average shares outstanding (basic) (in shares)	246,100,000	244,700,000
Common stock equivalents (in shares)	1,800,000	2,200,000
Incremental shares from assumed conversion of convertible debentures (in shares)	0	5,200,000
Diluted shares (in shares)	247,900,000	252,100,000
Basic earnings per share (in dollars per share)	$ 0.54	$ 0.45
Diluted earnings per share (in dollars per share)	$ 0.54	$ 0.43
Interest rate percentage on Convertible Debentures included in calculation of diluted earnings per share, prior period (in hundredths)		2.50%
Treasury shares acquired (in shares)	57,200	0
Average cost of treasury shares acquired (in dollars per share)	$ 55.38
Treasury shares issued in satisfaction of stock option exercises and vesting of stock units (in shares)	810,249	1,085,933