Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (1,534.5)	$ (919.0)
Investment income	85.5	49.0
Tax expense (benefit)	339.2	185.1
Net Income (Loss)	(1,195.3)	(733.9)
Reclassified out of AOCI	(20.2)	23.1
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	(20.4)	23.2
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(24.2)	29.9
Tax expense (benefit)	3.8	(6.7)
Net Income (Loss)	(20.4)	23.2
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	(0.9)	(0.8)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(1.2)	(1.1)
Tax expense (benefit)	0.3	0.3
Net Income (Loss)	(0.9)	(0.8)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(8,235.0)	(8,050.1)
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	8.8	0.2
Cost of sales and revenues	(7.4)
Interest and other (income) expenses, net	(1.1)	(0.7)
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	2.6	2.2
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income) expenses, net	1.2	0.7
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income) expenses, net	0.3	0.3
Truck, Parts and Other | Settlement loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	1.5	1.0
Tax expense (benefit)	(0.4)	(0.3)
Net Income (Loss)	1.1	0.7
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	(2.6)	0.8
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	$ (24.5)	$ 27.4