Net Income Per Common Share (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,383,991		208,369,188		208,205,469	208,241,714
Net Income	$ 396.2	$ 389.3	$ 368.1	$ 375.1	$ 785.5	$ 743.2
Less: Dividends on Preferred Stock	4.7		4.7		20.9	20.9
Net Income Applicable to Common Stock	391.5		363.4		764.6	722.3
Less: Earnings Allocated to Participating Securities	3.2		3.2		6.3	6.9
Earnings Allocated to Common Shares Outstanding	$ 388.3		$ 360.2		$ 758.3	$ 715.4
Basic Net Income Per Common Share (in dollars per share)	$ 1.86		$ 1.73		$ 3.64	$ 3.44
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,383,991		208,369,188		208,205,469	208,241,714
Plus: Dilutive Effect of Share-based Compensation (in shares)	494,359		768,902		638,465	801,084
Average Common and Potential Common Shares (in shares)	208,878,350		209,138,090		208,843,934	209,042,798
Earnings Allocated to Common and Potential Common Shares	$ 388.3		$ 360.2		$ 758.3	$ 715.4
Diluted Net Income Per Common Share (in dollars per share)	$ 1.86		$ 1.72		$ 3.63	$ 3.42
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0		0		0	0