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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic	227,535,701	233,149,020	228,077,395	233,264,453
Net Income	$ 260.7	$ 269.2	$ 502.5	$ 499.9
Less: Dividends on Preferred Stock	5.8	5.8	11.7	11.7
Net Income Applicable to Common Stock	254.9	263.4	490.8	488.2
Less: Earnings Allocated to Participating Securities	4.7	4.1	8.8	7.8
Net Income (Loss) Allocated to Common Stockholders, Basic	$ 250.2	$ 259.3	$ 482.0	$ 480.4
Basic Net Income Per Common Share (in dollars per share)	$ 1.10	$ 1.11	$ 2.11	$ 2.06
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding — Basic	227,535,701	233,149,020	228,077,395	233,264,453
Plus: Dilutive Effect of Share-based Compensation	1,661,312	2,083,815	1,510,957	1,996,158
Average Common and Potential Common Shares	229,197,013	235,232,835	229,588,352	235,260,611
Earnings Allocated to Common and Potential Common Shares	$ 250.1	$ 259.3	$ 482.0	$ 480.4
Diluted Net Income Per Common Share (in dollars per share)	$ 1.09	$ 1.10	$ 2.10	$ 2.04
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	331,233	0	1,557,641	748,268