Net Income Per Common Share (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,112,824	208,881,131
Net Income	$ 375.1	$ 360.6
Less: Dividends on Preferred Stock	16.2	30.5
Net Income Applicable to Common Stock	358.9	330.1
Less: Earnings Allocated to Participating Securities	3.7	3.9
Earnings Allocated to Common Shares Outstanding	$ 355.2	$ 326.2
Basic Net Income Per Common Share (in dollars per share)	$ 1.71	$ 1.56
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,112,824	208,881,131
Plus: Dilutive Effect of Share-based Compensation (in shares)	833,624	935,691
Diluted (in shares)	208,946,448	209,816,822
Earnings Allocated to Common and Potential Common Shares	$ 355.2	$ 326.2
Diluted Net Income Per Common Share (in dollars per share)	$ 1.70	$ 1.55
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0