Net Income Per Common Share (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,024,962	208,112,824
Net Income	$ 389.3	$ 375.1
Less: Dividends on Preferred Stock	16.2	16.2
Net Income Applicable to Common Stock	373.1	358.9
Less: Earnings Allocated to Participating Securities	3.1	3.7
Earnings Allocated to Common Shares Outstanding	$ 370.0	$ 355.2
Basic Net Income Per Common Share (in dollars per share)	$ 1.78	$ 1.71
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,024,962	208,112,824
Plus: Dilutive Effect of Share-based Compensation (in shares)	784,171	833,624
Average Common and Potential Common Shares (in shares)	208,809,133	208,946,448
Earnings Allocated to Common and Potential Common Shares	$ 370.0	$ 355.2
Diluted Net Income Per Common Share (in dollars per share)	$ 1.77	$ 1.70
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0