Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,106,190			213,176,809			208,351,088	215,832,347
Net Income	$ 294.5	$ 313.3	$ 360.6	$ 384.6	$ 389.4	$ 347.1	$ 968.4	$ 1,121.1
Less: Dividends on Preferred Stock	16.2			17.4			51.5	40.6
Net Income Applicable to Common Stock	278.3			367.2			916.9	1,080.5
Less: Earnings Allocated to Participating Securities	3.3			4.5			10.6	12.6
Earnings Allocated to Common Shares Outstanding	$ 275.0			$ 362.7			$ 906.3	$ 1,067.9
Basic Net Income Per Common Share (in dollars per share)	$ 1.32			$ 1.70			$ 4.35	$ 4.95
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	208,106,190			213,176,809			208,351,088	215,832,347
Plus: Dilutive Effect of Share-based Compensation (in shares)	582,304			1,029,767			672,243	1,031,216
Diluted (in shares)	208,688,494			214,206,576			209,023,331	216,863,563
Earnings Allocated to Common and Potential Common Shares	$ 275.0			$ 362.7			$ 906.3	$ 1,067.9
Diluted Net Income Per Common Share (in dollars per share)	$ 1.32			$ 1.69			$ 4.34	$ 4.92
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