Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	237,208,151	239,167,559
Net Income Applicable to Common Stock	$ 181.4	$ 164.0
Less: Earnings Allocated to Participating Securities	2.9	2.5
Earnings Allocated to Common Shares Outstanding	178.5	161.5
Basic Net Income Per Common Share	$ 0.75	$ 0.68
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	237,208,151	239,167,559
Plus: Dilutive Effect of Share-based Compensation	1,842,563	1,021,656
Average Common and Potential Common Shares	239,050,714	240,189,215
Earnings Allocated to Common and Potential Common Shares	$ 178.5	$ 161.5
Diluted Net Income Per Common Share	$ 0.75	$ 0.67