Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	235,701,076	239,930,074	236,301,789	239,614,868
Net Income	$ 204.5	$ 206.5	$ 567.8	$ 561.6
Less: Dividends on Preferred Stock
Net Income Applicable to Common Stock	204.5	206.5	567.8	561.6
Less: Earnings Allocated to Participating Securities	3.5	3.5	9.5	9.1
Earnings Allocated to Common Shares Outstanding	201.0	203.0	558.3	552.5
Basic Net Income Per Common Share	$ 0.85	$ 0.85	$ 2.36	$ 2.31
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	235,701,076	239,930,074	236,301,789	239,614,868
Plus: Dilutive Effect of Share-based Compensation	2,036,053	1,400,578	1,873,907	1,242,829
Average Common and Potential Common Shares	237,737,129	241,330,652	238,175,696	240,857,697
Earnings Allocated to Common and Potential Common Shares	$ 201.0	$ 203.0	$ 558.4	$ 552.5
Diluted Net Income Per Common Share	$ 0.84	$ 0.84	$ 2.34	$ 2.29