Net Income per Common Share - Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
BASIC NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	207,248,094	208,309,331	208,075,522
Net Income	$ 1,107.3	$ 1,336.0	$ 1,545.3
Less: Dividends on Preferred Stock	41.8	41.8	41.8
Net Income Applicable to Common Stock	1,065.5	1,294.2	1,503.5
Less: Earnings Allocated to Participating Securities	11.6	11.8	12.9
Earnings Allocated to Common Shares Outstanding	$ 1,053.9	$ 1,282.4	$ 1,490.6
Basic Net Income Per Common Share (in dollars per share)	$ 5.09	$ 6.16	$ 7.16
DILUTED NET INCOME PER COMMON SHARE
Average Number of Common Shares Outstanding (in shares)	207,248,094	208,309,331	208,075,522
Plus Dilutive Effect of Share-based Compensation (in shares)	315,652	557,933	823,708
Average Common and Potential Common Shares (in shares)	207,563,746	208,867,264	208,899,230
Earnings Allocated to Common and Potential Common Shares	$ 1,053.9	$ 1,282.4	$ 1,490.6
Diluted Net Income Per Common Share (in dollars per share)	$ 5.08	$ 6.14	$ 7.14
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive (in shares)	100,000	0	0