Net Income Per Common Share (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	207,021,554			208,400,135			207,611,420	208,271,070
Net Income	$ 327.8	$ 331.8	$ 334.6	$ 394.8	$ 396.2	$ 389.3	$ 994.2	$ 1,180.3
Less: Dividends on Preferred Stock	16.2			16.2			37.1	37.1
Net Income Applicable to Common Stock	311.6			378.6			957.1	1,143.2
Less: Earnings Allocated to Participating Securities	3.1			3.2			9.7	9.5
Earnings Allocated to Common Shares Outstanding	$ 308.5			$ 375.4			$ 947.4	$ 1,133.7
Basic Net Income Per Common Share (in dollars per share)	$ 1.49			$ 1.80			$ 4.56	$ 5.44
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	207,021,554			208,400,135			207,611,420	208,271,070
Plus: Dilutive Effect of Share-based Compensation (in shares)	231,438			488,437			316,300	587,906
Average Common and Potential Common Shares (in shares)	207,252,992			208,888,572			207,927,720	208,858,976
Earnings Allocated to Common and Potential Common Shares	$ 308.5			$ 375.4			$ 947.3	$ 1,133.7
Diluted Net Income Per Common Share (in dollars per share)	$ 1.49			$ 1.80			$ 4.56	$ 5.43
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	48,596			0			129,239	0