Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Balance Sheet (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net		$ 1,976.0
Retained earnings	$ 13,760.8	12,025.8
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	2,382.1	1,976.0
Accounts payable, accrued expenses and other	4,381.8
Retained earnings	13,760.8
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	2,138.7
Accounts payable, accrued expenses and other	4,321.9
Retained earnings	13,577.3
Adjustment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net		207.7
Retained earnings		$ 156.6
Adjustment | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	243.4
Accounts payable, accrued expenses and other	59.9
Retained earnings	$ 183.5