Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Balance Sheet (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net		$ 1,976.0
Retained earnings	$ 12,507.9	12,025.8
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	2,222.1	1,976.0
Accounts payable, accrued expenses and other	4,609.6
Retained earnings	12,507.9
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	2,003.1
Accounts payable, accrued expenses and other	4,555.7
Retained earnings	12,342.8
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	219.0
Accounts payable, accrued expenses and other	53.9
Retained earnings	$ 165.1