Net Income Per Common Share Computations (Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	229,059,540	228,619,089
Net Income	$ 276.1	$ 245.4
Less: Dividends on Preferred Stock	20.7	5.9
Net Income Applicable to Common Stock	255.4	239.5
Less: Earnings Allocated to Participating Securities	4.5	4.1
Earnings Allocated to Common Shares Outstanding	$ 250.9	$ 235.4
Basic Earnings per Share	$ 1.10	$ 1.03
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic (in shares)	229,059,540	228,619,089
Plus: Dilutive Effect of Share-based Compensation (in shares)	1,571,336	1,178,856
Diluted (in shares)	230,630,876	229,797,945
Earnings Allocated to Common and Potential Common Shares	$ 250.9	$ 235.5
Diluted Earnings per Share	$ 1.09	$ 1.03
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	468,381	1,796,320