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, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	207,638,671		208,383,991		207,911,242	208,205,469
Net Income	$ 331.8	$ 334.6	$ 396.2	$ 389.3	$ 666.4	$ 785.5
Less: Dividends on Preferred Stock	4.7		4.7		20.9	20.9
Net Income Applicable to Common Stock	327.1		391.5		645.5	764.6
Less: Earnings Allocated to Participating Securities	3.4		3.2		6.6	6.3
Earnings Allocated to Common Shares Outstanding	$ 323.7		$ 388.3		$ 638.9	$ 758.3
Basic Net Income Per Common Share (in dollars per share)	$ 1.56		$ 1.86		$ 3.07	$ 3.64
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	207,638,671		208,383,991		207,911,242	208,205,469
Plus: Dilutive Effect of Share-based Compensation (in shares)	177,344		494,359		359,435	638,465
Average Common and Potential Common Shares (in shares)	207,816,015		208,878,350		208,270,677	208,843,934
Earnings Allocated to Common and Potential Common Shares	$ 323.6		$ 388.3		$ 638.8	$ 758.3
Diluted Net Income Per Common Share (in dollars per share)	$ 1.56		$ 1.86		$ 3.07	$ 3.63
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