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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	222,442,797	228,010,866	224,098,656	228,751,804
Net Income	$ 374.5	$ 298.4	$ 1,146.5	$ 842.4
Less: Dividends on Preferred Stock	17.3	17.3	40.5	43.9
Net Income Applicable to Common Stock	357.2	281.1	1,106.0	798.5
Less: Earnings Allocated to Participating Securities	4.1	4.4	14.3	13.3
Earnings Allocated to Common Shares Outstanding	$ 353.1	$ 276.7	$ 1,091.7	$ 785.2
Basic Earnings per Share (in dollars per share)	$ 1.59	$ 1.21	$ 4.87	$ 3.43
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	222,442,797	228,010,866	224,098,656	228,751,804
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,417,217	1,302,779	1,395,979	1,437,485
Diluted (in shares)	223,860,014	229,313,645	225,494,635	230,189,289
Earnings Allocated to Common and Potential Common Shares	$ 353.1	$ 276.7	$ 1,091.8	$ 785.2
Diluted Earnings per Share (in dollars per share)	$ 1.58	$ 1.20	$ 4.84	$ 3.41
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	154,411