Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	218,236,802	225,681,167
Net Income	$ 347.1	$ 381.6
Less: Dividends on Preferred Stock	17.3	17.3
Net Income Applicable to Common Stock	329.8	364.3
Less: Earnings Allocated to Participating Securities	4.3	5.2
Earnings Allocated to Common Shares Outstanding	$ 325.5	$ 359.1
Basic Earnings per Share (in dollars per share)	$ 1.49	$ 1.59
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding (in shares)	218,236,802	225,681,167
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,033,413	1,366,352
Diluted (in shares)	219,270,215	227,047,519
Earnings Allocated to Common and Potential Common Shares	$ 325.4	$ 359.2
Diluted Earnings per Share (in dollars per share)	$ 1.48	$ 1.58
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0