Net Income per Common Share - Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	208,319,412	214,525,547	223,148,335
Net Income	$ 1,209.3	$ 1,492.2	$ 1,556.4
Less: Dividends on Preferred Stock	56.2	46.4	46.4
Net Income Applicable to Common Stock	1,153.1	1,445.8	1,510.0
Less: Earnings Allocated to Participating Securities	12.1	16.9	20.1
Earnings Allocated to Common Shares Outstanding	$ 1,141.0	$ 1,428.9	$ 1,489.9
Basic Net Income Per Common Share (in dollars per share)	$ 5.48	$ 6.66	$ 6.68
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	208,319,412	214,525,547	223,148,335
Plus Dilutive Effect of Share-based Compensation (in shares)	688,574	1,075,602	1,339,991
Average Common and Potential Common Shares (in shares)	209,007,986	215,601,149	224,488,326
Earnings Allocated to Common and Potential Common Shares	$ 1,141.1	$ 1,428.9	$ 1,490.0
Diluted Net Income Per Common Share (in dollars per share)	$ 5.46	$ 6.63	$ 6.64
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	0	0	0