Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ 4,176.2		$ 13,525.3
Income taxes	$ 210.3	106.7	$ 582.9	389.0
Net income	$ 769.4	$ 380.5	$ 2,090.3	$ 1,346.8
Net Income Per Share
Basic	$ 2.21	$ 1.09	$ 6.00	$ 3.87
Diluted	$ 2.21	$ 1.09	$ 5.99	$ 3.87
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,689.3		$ 16,785.8
Income taxes	210.3		582.9
Net income	$ 769.4		$ 2,090.3
Net Income Per Share
Basic	$ 2.21		$ 6.00
Diluted	$ 2.21		$ 5.99
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ 5,699.1		$ 16,821.5
Income taxes	207.9		574.1
Net income	$ 762.0		$ 2,063.4
Net Income Per Share
Basic	$ 2.19		$ 5.92
Diluted	$ 2.18		$ 5.91
Adjustment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues		$ (3.9)		$ (8.2)
Income taxes		1.1		2.1
Net income		$ 2.8		$ 6.1
Adjustment | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cost of sales and revenues	$ (9.8)		$ (35.7)
Income taxes	2.4		8.8
Net income	$ 7.4		$ 26.9
Net Income Per Share
Basic	$ 0.02		$ 0.08
Diluted	$ 0.03		$ 0.08