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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	224,207,590	229,196,531	224,940,308	229,128,414
Net Income	$ 390.4	$ 267.9	$ 772.0	$ 544.0
Less: Dividends on Preferred Stock	5.9	5.9	23.2	26.6
Net Income Applicable to Common Stock	384.5	262.0	748.8	517.4
Less: Earnings Allocated to Participating Securities	5.0	4.4	10.2	8.9
Earnings Allocated to Common Shares Outstanding	$ 379.5	$ 257.6	$ 738.6	$ 508.5
Basic Earnings per Share (in dollars per share)	$ 1.69	$ 1.12	$ 3.28	$ 2.22
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	224,207,590	229,196,531	224,940,308	229,128,414
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,403,807	1,441,292	1,385,184	1,505,954
Diluted (in shares)	225,611,397	230,637,823	226,325,492	230,634,368
Earnings Allocated to Common and Potential Common Shares	$ 379.5	$ 257.6	$ 738.6	$ 508.5
Diluted Earnings per Share (in dollars per share)	$ 1.68	$ 1.12	$ 3.26	$ 2.21
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	232,897