Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	241,090,093	242,126,162
Net Income	$ 161.2	$ 151.0
Net Income Applicable to Common Stock	161.2	151.0
Less: Earnings Allocated to Participating Securities	2.1	1.7
Earnings Allocated to Common Shares Outstanding	159.1	149.3
Basic Net Income Per Common Share	$ 0.66	$ 0.62
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	241,090,093	242,126,162
Plus Dilutive Effect of Share-based Compensation	466,003	843,467
Average Common and Potential Common Shares	241,556,096	242,969,629
Earnings Allocated to Common and Potential Common Shares	$ 160.0	$ 149.3
Diluted Net Income Per Common Share	$ 0.66	$ 0.61