Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	229,196,531	227,535,701	229,128,414	228,077,395
Net Income	$ 267.9	$ 263.0	$ 544.0	$ 508.4
Less: Dividends on Preferred Stock	5.9	5.8	26.6	11.7
Net Income Applicable to Common Stock	262.0	257.2	517.4	496.7
Less: Earnings Allocated to Participating Securities	4.4	4.8	8.9	8.9
Earnings Allocated to Common Shares Outstanding	$ 257.6	$ 252.4	$ 508.5	$ 487.8
Basic Earnings per Share	$ 1.12	$ 1.11	$ 2.22	$ 2.14
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	229,196,531	227,535,701	229,128,414	228,077,395
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,441,292	1,743,839	1,505,954	1,461,348
Diluted (in shares)	230,637,823	229,279,540	230,634,368	229,538,743
Earnings Allocated to Common and Potential Common Shares	$ 257.6	$ 252.4	$ 508.5	$ 487.9
Diluted Earnings per Share	$ 1.12	$ 1.10	$ 2.21	$ 2.13
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	331,233	232,897	1,557,641