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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	228,010,866	226,540,086	228,751,804	227,561,218
Net Income	$ 298.4	$ 257.6	$ 842.4	$ 766.0
Less: Dividends on Preferred Stock	17.3	5.9	43.9	17.6
Net Income Applicable to Common Stock	281.1	251.7	798.5	748.4
Less: Earnings Allocated to Participating Securities	4.4	4.8	13.3	13.7
Earnings Allocated to Common Shares Outstanding	$ 276.7	$ 246.9	$ 785.2	$ 734.7
Basic Earnings per Share	$ 1.21	$ 1.09	$ 3.43	$ 3.23
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	228,010,866	226,540,086	228,751,804	227,561,218
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,302,779	1,515,109	1,437,485	1,479,400
Diluted (in shares)	229,313,645	228,055,195	230,189,289	229,040,618
Earnings Allocated to Common and Potential Common Shares	$ 276.7	$ 247.0	$ 785.2	$ 734.8
Diluted Earnings per Share	$ 1.20	$ 1.08	$ 3.41	$ 3.21
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	334,184	154,411	1,480,118