Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	$ 5,891.0	$ 6,673.8
Interest and other borrowing expenses	190.6	160.0
Total Income Before Income Taxes	643.1	1,534.5
Investment income	83.8	85.5
Tax (benefit) expense	138.0	339.2
Net Income	505.1	1,195.3
Reclassified out of AOCI	54.5	(20.2)
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	53.8	(20.4)
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Income Before Income Taxes	68.2	(24.2)
Tax (benefit) expense	(14.4)	3.8
Net Income	53.8	(20.4)
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	(0.1)	(0.9)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.2)	(1.2)
Tax (benefit) expense	0.1	0.3
Net Income	(0.1)	(0.9)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(6,913.7)	(8,235.0)
Truck, Parts and Other | Derivative Contracts | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(2.8)	8.8
Cost of sales and revenues	(9.5)	(7.4)
Interest and other expenses (income), net	12.4	(1.1)
Truck, Parts and Other | Derivative Contracts | Commodity contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	(0.1)	2.6
Truck, Parts and Other | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other expenses (income), net	0.7	1.2
Truck, Parts and Other | Prior Service Costs | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other expenses (income), net	0.3	0.3
Truck, Parts and Other | Settlement loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Income Before Income Taxes	1.0	1.5
Tax (benefit) expense	(0.2)	(0.4)
Net Income	0.8	1.1
Financial Services | Derivative Contracts | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	5.8	(2.6)
Financial Services | Derivative Contracts | Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	$ 62.4	$ (24.5)