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, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	203,306,236		207,638,671		203,967,516	207,911,242
Net Income	$ 896.1	$ 214.7	$ 331.8	$ 334.6	$ 1,110.8	$ 666.4
Less: Dividends on Preferred Stock	4.7		4.7		20.9	20.9
Net Income Applicable to Common Stock	891.4		327.1		1,089.9	645.5
Less: Earnings Allocated to Participating Securities	7.1		3.4		9.5	6.6
Earnings Allocated to Common Shares Outstanding	$ 884.3		$ 323.7		$ 1,080.4	$ 638.9
Basic Net Income Per Common Share (in dollars per share)	$ 4.35		$ 1.56		$ 5.30	$ 3.07
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	203,306,236		207,638,671		203,967,516	207,911,242
Plus: Dilutive Effect of Share-based Compensation (in shares)	432,434		177,344		469,241	359,435
Average Common and Potential Common Shares (in shares)	203,738,670		207,816,015		204,436,757	208,270,677
Earnings Allocated to Common and Potential Common Shares	$ 884.3		$ 323.6		$ 1,080.4	$ 638.8
Diluted Net Income Per Common Share (in dollars per share)	$ 4.34		$ 1.56		$ 5.28	$ 3.07
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0		0		100,000	100,000