Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2024		Jun. 30, 2023	Jun. 30, 2024		Jun. 30, 2023
Numerator
Net Income	$ 389.5		$ 392.9	$ 784.7		$ 751.2
Denominator
Weighted Average Common Shares - Basic	189,894,100		197,180,400	191,302,500		197,641,300
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	435,200		956,100	576,000.0		1,182,500
Weighted Average Common Shares - Assuming Dilution	190,329,300		198,136,500	191,878,500		198,823,800
Net Income Per Common Share
Basic	$ 2.05		$ 1.99	$ 4.10		$ 3.80
Assuming Dilution	$ 2.05		$ 1.98	$ 4.09		$ 3.78
Cost of Shares Repurchased	$ 179.8	[1]	$ 47.0	$ 302.8	[1]	$ 100.6

[1] 1Includes a de minimis amount of commissions for the three and six months ended June 30, 2024 and $0.1 million of commissions for the three and six months ended June 30, 2023. Also includes $1.7 million and $2.8 million of excise tax for the three and six months ended June 30, 2024, respectively, and $0.4 million and $0.5 million of excise tax for the three and six months ended June 30, 2023, respectively.