Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,068,635		216,139,033		208,474,883	217,182,123
Net Income	$ 313.3	$ 360.6	$ 389.4	$ 347.1	$ 673.9	$ 736.5
Less: Dividends on Preferred Stock	4.8		5.9		35.3	23.2
Net Income Applicable to Common Stock	308.5		383.5		638.6	713.3
Less: Earnings Allocated to Participating Securities	3.4		3.8		7.3	8.1
Earnings Allocated to Common Shares Outstanding	$ 305.1		$ 379.7		$ 631.3	$ 705.2
Basic Net Income Per Common Share (in dollars per share)	$ 1.47		$ 1.76		$ 3.03	$ 3.25
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,068,635		216,139,033		208,474,883	217,182,123
Plus: Dilutive Effect of Share-based Compensation (in shares)	499,722		1,030,506		717,707	1,031,952
Diluted (in shares)	208,568,357		217,169,539		209,192,590	218,214,075
Earnings Allocated to Common and Potential Common Shares	$ 305.2		$ 379.8		$ 631.3	$ 705.2
Diluted Net Income Per Common Share (in dollars per share)	$ 1.46		$ 1.75		$ 3.02	$ 3.23
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