Computations of Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in dollars per share)	227,580,584	232,279,849	235,829,790
Net Income	$ 1,032.5	$ 973.8	$ 811.8
Less: Dividends on Preferred Stock	23.4	23.4	9.5
Net Income Applicable to Common Stock	1,009.1	950.4	802.3
Less: Earnings Allocated to Participating Securities	18.7	15.4	13.3
Earnings Allocated to Common Shares Outstanding	$ 990.4	$ 935.0	$ 789.0
Basic Net Income Per Common Share (in dollars per share)	$ 4.35	$ 4.03	$ 3.34
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in dollars per share)	227,580,584	232,279,849	235,829,790
Plus Dilutive Effect of Share-based Compensation (in shares)	1,570,822	1,941,880	1,890,465
Average Common and Potential Common Shares (in shares)	229,151,406	234,221,729	237,720,255
Earnings Allocated to Common and Potential Common Shares	$ 990.4	$ 935.0	$ 789.0
Diluted Net Income Per Common Share (in dollars per share)	$ 4.32	$ 3.99	$ 3.32
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	0	371,059	1,517,588