Computations of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	233,149,020	236,012,703	233,264,453	236,607,125
Net Income	$ 269.2	$ 181.9	$ 499.9	$ 363.3
Less: Dividends on Preferred Stock	5.8		11.7
Net Income Applicable to Common Stock	263.4	181.9	488.2	363.3
Less: Earnings Allocated to Participating Securities	4.1	3.1	7.8	6.0
Earnings Allocated to Common Shares Outstanding	$ 259.3	$ 178.8	$ 480.4	$ 357.3
Basic Net Income Per Common Share	$ 1.11	$ 0.76	$ 2.06	$ 1.51
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	233,149,020	236,012,703	233,264,453	236,607,125
Plus: Dilutive Effect of Share-based Compensation	2,083,815	1,740,976	1,996,158	1,791,489
Average Common and Potential Common Shares	235,232,835	237,753,679	235,260,611	238,398,614
Earnings Allocated to Common and Potential Common Shares	$ 259.3	$ 178.8	$ 480.4	$ 357.3
Diluted Net Income Per Common Share	$ 1.10	$ 0.75	$ 2.04	$ 1.50