Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,881,131	218,236,802
Net Income	$ 360.6	$ 347.1
Less: Dividends on Preferred Stock	30.5	17.3
Net Income Applicable to Common Stock	330.1	329.8
Less: Earnings Allocated to Participating Securities	3.9	4.3
Earnings Allocated to Common Shares Outstanding	$ 326.2	$ 325.5
Basic Earnings per Share (in dollars per share)	$ 1.56	$ 1.49
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,881,131	218,236,802
Plus: Dilutive Effect of Share-based Compensation (in shares)	935,691	1,033,413
Diluted (in shares)	209,816,822	219,270,215
Earnings Allocated to Common and Potential Common Shares	$ 326.2	$ 325.4
Diluted Earnings per Share (in dollars per share)	$ 1.55	$ 1.48
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0