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, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,369,188		208,068,635		208,241,714	208,474,883
Net Income	$ 368.1	$ 375.1	$ 313.3	$ 360.6	$ 743.2	$ 673.9
Less: Dividends on Preferred Stock	4.7		4.8		20.9	35.3
Net Income Applicable to Common Stock	363.4		308.5		722.3	638.6
Less: Earnings Allocated to Participating Securities	3.2		3.4		6.9	7.3
Earnings Allocated to Common Shares Outstanding	$ 360.2		$ 305.1		$ 715.4	$ 631.3
Basic Net Income Per Common Share (in dollars per share)	$ 1.73		$ 1.47		$ 3.44	$ 3.03
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,369,188		208,068,635		208,241,714	208,474,883
Plus: Dilutive Effect of Share-based Compensation (in shares)	768,902		499,722		801,084	717,707
Diluted (in shares)	209,138,090		208,568,357		209,042,798	209,192,590
Earnings Allocated to Common and Potential Common Shares	$ 360.2		$ 305.2		$ 715.4	$ 631.3
Diluted Net Income Per Common Share (in dollars per share)	$ 1.72		$ 1.46		$ 3.42	$ 3.02
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0					0