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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	226,540,086	232,231,720	227,561,218	232,916,425
Net Income	$ 257.6	$ 234.6	$ 766.0	$ 734.5
Less: Dividends on Preferred Stock	5.9	5.8	17.6	17.5
Net Income Applicable to Common Stock	251.7	228.8	748.4	717.0
Less: Earnings Allocated to Participating Securities	4.8	3.8	13.7	11.6
Net Income (Loss) Allocated to Common Stockholders, Basic	$ 246.9	$ 225.0	$ 734.7	$ 705.4
Basic Net Income Per Common Share (in dollars per share)	$ 1.09	$ 0.97	$ 3.23	$ 3.03
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	226,540,086	232,231,720	227,561,218	232,916,425
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,515,109	1,931,470	1,479,400	1,974,357
Average Common and Potential Common Shares (in shares)	228,055,195	234,163,190	229,040,618	234,890,782
Earnings Allocated to Common and Potential Common Shares	$ 247.0	$ 225.0	$ 734.8	$ 705.4
Diluted Net Income Per Common Share (in dollars per share)	$ 1.08	$ 0.96	$ 3.21	$ 3.00
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	334,184	0	1,480,118	496,104