Compare Currently Reported Amounts and Reported Under LIFO in Consolidated Balance Sheet (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net		$ 1,976.0
Retained earnings	$ 13,109.9	12,025.8
Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	2,232.2	1,976.0
Accounts payable, accrued expenses and other	4,537.0
Retained earnings	13,109.9
Pro Forma Under LIFO | Truck, Parts and Other
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Inventories, net	1,998.6
Accounts payable, accrued expenses and other	4,479.5
Retained earnings	12,933.8
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	233.6
Accounts payable, accrued expenses and other	57.5
Retained earnings	$ 176.1