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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	240,237,014	240,991,491	240,740,803	241,529,793
Net Income Applicable to Common Stock	$ 178.8	$ 170.4	$ 519.6	$ 473.4
Less: Earnings Allocated to Participating Securities	2.8	2.0	7.6	5.5
Earnings Allocated to Common Shares Outstanding	176.0	168.4	512.0	467.9
Basic Net Income Per Common Share	$ 0.73	$ 0.70	$ 2.13	$ 1.94
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	240,237,014	240,991,491	240,740,803	241,529,793
Plus Dilutive Effect of Share-based Compensation	460,048	202,502	464,383	488,957
Average Common and Potential Common Shares	240,697,062	241,193,993	241,205,186	242,018,750
Earnings Allocated to Common and Potential Common Shares	$ 176.0	$ 168.4	$ 512.0	$ 467.9
Diluted Net Income Per Common Share	$ 0.73	$ 0.70	$ 2.12	$ 1.93