Computations of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in dollars per share)	232,279,849	235,829,790	239,265,313
Net Income	$ 973.8	$ 811.8	$ 731.3
Less: Dividends on Preferred Stock	23.4	9.5	0.0
Net Income Applicable to Common Stock	950.4	802.3	731.3
Less: Earnings Allocated to Participating Securities	15.4	13.3	11.9
Earnings Allocated to Common Shares Outstanding	$ 935.0	$ 789.0	$ 719.4
Basic Net Income Per Common Share (in dollars per share)	$ 4.03	$ 3.34	$ 3.01
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in dollars per share)	232,279,849	235,829,790	239,265,313
Plus Dilutive Effect of Share-based Compensation (in shares)	1,941,880	1,890,465	1,289,527
Average Common and Potential Common Shares (in shares)	234,221,729	237,720,255	240,554,840
Earnings Allocated to Common and Potential Common Shares	$ 935.0	$ 789.0	$ 719.5
Diluted Net Income Per Common Share (in dollars per share)	$ 3.99	$ 3.32	$ 2.99
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	371,059	1,517,588	3,498,894