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, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2022	Sep. 30, 2021
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,400,135			208,116,009			208,271,070	208,199,352
Net Income	$ 394.8	$ 396.2	$ 389.3	$ 395.7	$ 368.1	$ 375.1	$ 1,180.3	$ 1,138.9
Less: Dividends on Preferred Stock	16.2			16.2			37.1	37.1
Net Income Applicable to Common Stock	378.6			379.5			1,143.2	1,101.8
Less: Earnings Allocated to Participating Securities	3.2			3.0			9.5	9.9
Earnings Allocated to Common Shares Outstanding	$ 375.4			$ 376.5			$ 1,133.7	$ 1,091.9
Basic Net Income Per Common Share (in dollars per share)	$ 1.80			$ 1.81			$ 5.44	$ 5.24
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,400,135			208,116,009			208,271,070	208,199,352
Plus: Dilutive Effect of Share-based Compensation (in shares)	488,437			807,297			587,906	803,178
Average Common and Potential Common Shares (in shares)	208,888,572			208,923,306			208,858,976	209,002,530
Earnings Allocated to Common and Potential Common Shares	$ 375.4			$ 376.5			$ 1,133.7	$ 1,091.9
Diluted Net Income Per Common Share (in dollars per share)	$ 1.80			$ 1.80			$ 5.43	$ 5.22
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