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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	216,139,033	224,207,590	217,182,123	224,940,308
Net Income	$ 389.4	$ 390.4	$ 736.5	$ 772.0
Less: Dividends on Preferred Stock	5.9	5.9	23.2	23.2
Net Income Applicable to Common Stock	383.5	384.5	713.3	748.8
Less: Earnings Allocated to Participating Securities	3.8	5.0	8.1	10.2
Earnings Allocated to Common Shares Outstanding	$ 379.7	$ 379.5	$ 705.2	$ 738.6
Basic Earnings per Share (in dollars per share)	$ 1.76	$ 1.69	$ 3.25	$ 3.28
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	216,139,033	224,207,590	217,182,123	224,940,308
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,030,506	1,403,807	1,031,952	1,385,184
Diluted (in shares)	217,169,539	225,611,397	218,214,075	226,325,492
Earnings Allocated to Common and Potential Common Shares	$ 379.8	$ 379.5	$ 705.2	$ 738.6
Diluted Earnings per Share (in dollars per share)	$ 1.75	$ 1.68	$ 3.23	$ 3.26
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	0