Computations of Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	239,738,592	240,900,839	239,454,653	240,995,466
Net Income Applicable to Common Stock	$ 191.1	$ 179.6	$ 355.1	$ 340.8
Less: Earnings Allocated to Participating Securities	3.2	2.7	5.6	4.8
Earnings Allocated to Common Shares Outstanding	187.9	176.9	349.5	336.0
Basic Net Income Per Common Share	$ 0.78	$ 0.73	$ 1.46	$ 1.39
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	239,738,592	240,900,839	239,454,653	240,995,466
Plus: Dilutive Effect of Share-based Compensation	1,302,089	467,143	1,162,647	466,573
Average Common and Potential Common Shares	241,040,681	241,367,982	240,617,300	241,462,039
Earnings Allocated to Common and Potential Common Shares	$ 187.9	$ 176.9	$ 349.5	$ 336.0
Diluted Net Income Per Common Share	$ 0.78	$ 0.73	$ 1.45	$ 1.39