Net Income per Common Share - Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	223,148,335	228,257,664	227,580,584
Net Income	$ 1,556.4	$ 1,199.0	$ 1,032.5
Less: Dividends on Preferred Stock	46.4	49.8	23.4
Net Income Applicable to Common Stock	1,510.0	1,149.2	1,009.1
Less: Earnings Allocated to Participating Securities	20.1	18.8	18.7
Earnings Allocated to Common Shares Outstanding	$ 1,489.9	$ 1,130.4	$ 990.4
Basic Net Income Per Common Share (in dollars per share)	$ 6.68	$ 4.95	$ 4.35
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	223,148,335	228,257,664	227,580,584
Plus Dilutive Effect of Share-based Compensation (in shares)	1,339,991	1,396,737	1,570,822
Average Common and Potential Common Shares (in shares)	224,488,326	229,654,401	229,151,406
Earnings Allocated to Common and Potential Common Shares	$ 1,490.0	$ 1,130.5	$ 990.4
Diluted Net Income Per Common Share (in dollars per share)	$ 6.64	$ 4.92	$ 4.32
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	0	115,491	1,108,067