Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	225,681,167	229,059,540
Net Income	$ 381.6	$ 276.1
Less: Dividends on Preferred Stock	17.3	20.7
Net Income Applicable to Common Stock	364.3	255.4
Less: Earnings Allocated to Participating Securities	5.2	4.5
Earnings Allocated to Common Shares Outstanding	$ 359.1	$ 250.9
Basic Earnings per Share (in dollars per share)	$ 1.59	$ 1.10
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	225,681,167	229,059,540
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,366,352	1,571,336
Diluted (in shares)	227,047,519	230,630,876
Earnings Allocated to Common and Potential Common Shares	$ 359.2	$ 250.9
Diluted Earnings per Share (in dollars per share)	$ 1.58	$ 1.09
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	468,381