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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	236,012,703	239,738,592	236,607,125	239,454,653
Net Income Applicable to Common Stock	$ 181.9	$ 191.1	$ 363.3	$ 355.1
Less: Earnings Allocated to Participating Securities	3.1	3.2	6.0	5.6
Earnings Allocated to Common Shares Outstanding	178.8	187.9	357.3	349.5
Basic Net Income Per Common Share	$ 0.76	$ 0.78	$ 1.51	$ 1.46
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	236,012,703	239,738,592	236,607,125	239,454,653
Plus: Dilutive Effect of Share-based Compensation	1,740,976	1,302,089	1,791,489	1,162,647
Average Common and Potential Common Shares	237,753,679	241,040,681	238,398,614	240,617,300
Earnings Allocated to Common and Potential Common Shares	$ 178.8	$ 187.9	$ 357.3	$ 349.5
Diluted Net Income Per Common Share	$ 0.75	$ 0.78	$ 1.50	$ 1.45