Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	228,619,089	233,381,168
Net Income	$ 241.8	$ 230.7
Less: Dividends on Preferred Stock	5.9	5.9
Net Income Applicable to Common Stock	235.9	224.8
Less: Earnings Allocated to Participating Securities	4.1	3.7
Earnings Allocated to Common Shares Outstanding	$ 231.8	$ 221.1
Basic Net Income Per Common Share (in dollars per share)	$ 1.01	$ 0.95
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	228,619,089	233,381,168
Plus: Dilutive Effect of Share-based Compensation	1,360,601	1,907,527
Average Common and Potential Common Shares	229,979,690	235,288,695
Earnings Allocated to Common and Potential Common Shares	$ 231.9	$ 221.1
Diluted Net Income Per Common Share (in dollars per share)	$ 1.01	$ 0.94
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	1,796,320	1,504,850