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, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,116,009			208,106,190			208,199,352	208,351,088
Net Income	$ 395.7	$ 368.1	$ 375.1	$ 294.5	$ 313.3	$ 360.6	$ 1,138.9	$ 968.4
Less: Dividends on Preferred Stock	16.2			16.2			37.1	51.5
Net Income Applicable to Common Stock	379.5			278.3			1,101.8	916.9
Less: Earnings Allocated to Participating Securities	3.0			3.3			9.9	10.6
Earnings Allocated to Common Shares Outstanding	$ 376.5			$ 275.0			$ 1,091.9	$ 906.3
Basic Net Income Per Common Share (in dollars per share)	$ 1.81			$ 1.32			$ 5.24	$ 4.35
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,116,009			208,106,190			208,199,352	208,351,088
Plus: Dilutive Effect of Share-based Compensation (in shares)	807,297			582,304			803,178	672,243
Diluted (in shares)	208,923,306			208,688,494			209,002,530	209,023,331
Earnings Allocated to Common and Potential Common Shares	$ 376.5			$ 275.0			$ 1,091.9	$ 906.3
Diluted Net Income Per Common Share (in dollars per share)	$ 1.80			$ 1.32			$ 5.22	$ 4.34
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0							0