Net Income Per Common Share (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,186,841	208,024,962
Net Income	$ 334.6	$ 389.3
Less: Dividends on Preferred Stock	16.2	16.2
Net Income Applicable to Common Stock	318.4	373.1
Less: Earnings Allocated to Participating Securities	3.2	3.1
Earnings Allocated to Common Shares Outstanding	$ 315.2	$ 370.0
Basic Net Income Per Common Share (in dollars per share)	$ 1.51	$ 1.78
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,186,841	208,024,962
Plus: Dilutive Effect of Share-based Compensation (in shares)	543,548	784,171
Average Common and Potential Common Shares (in shares)	208,730,389	208,809,133
Earnings Allocated to Common and Potential Common Shares	$ 315.2	$ 370.0
Diluted Net Income Per Common Share (in dollars per share)	$ 1.51	$ 1.77
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0