Net Income per Common Share - Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	228,257,664	227,580,584	232,279,849
Net Income	$ 1,199.0	$ 1,032.5	$ 973.8
Less: Dividends on Preferred Stock	49.8	23.4	23.4
Net Income Applicable to Common Stock	1,149.2	1,009.1	950.4
Less: Earnings Allocated to Participating Securities	18.8	18.7	15.4
Earnings Allocated to Common Shares Outstanding	$ 1,130.4	$ 990.4	$ 935.0
Basic Net Income Per Common Share (in dollars per share)	$ 4.95	$ 4.35	$ 4.03
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	228,257,664	227,580,584	232,279,849
Plus Dilutive Effect of Share-based Compensation (in shares)	1,396,737	1,570,822	1,941,880
Average Common and Potential Common Shares (in shares)	229,654,401	229,151,406	234,221,729
Earnings Allocated to Common and Potential Common Shares	$ 1,130.5	$ 990.4	$ 935.0
Diluted Net Income Per Common Share (in dollars per share)	$ 4.92	$ 4.32	$ 3.99
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	115,491	1,108,067	371,059