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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	232,231,720	235,701,076	232,916,425	236,301,789
Net Income	$ 234.6	$ 204.5	$ 734.5	$ 567.8
Less: Dividends on Preferred Stock	5.8	0.0	17.5	0.0
Net Income Applicable to Common Stock	228.8	204.5	717.0	567.8
Less: Earnings Allocated to Participating Securities	3.8	3.5	11.6	9.5
Earnings Allocated to Common Shares Outstanding	$ 225.0	$ 201.0	$ 705.4	$ 558.3
Basic Net Income Per Common Share	$ 0.97	$ 0.85	$ 3.03	$ 2.36
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	232,231,720	235,701,076	232,916,425	236,301,789
Plus: Dilutive Effect of Share-based Compensation	1,931,470	2,036,053	1,974,357	1,873,907
Average Common and Potential Common Shares	234,163,190	237,737,129	234,890,782	238,175,696
Earnings Allocated to Common and Potential Common Shares	$ 225.0	$ 201.0	$ 705.4	$ 558.4
Diluted Net Income Per Common Share	$ 0.96	$ 0.84	$ 3.00	$ 2.34
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	0	1,071,654	496,104	1,671,849