Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (1,256.3)	$ (1,589.4)	$ (4,251.6)	$ (4,076.9)
Investment income	108.7	80.8	290.0	192.5
Tax expense (benefit)	284.2	360.9	961.6	893.4
Net Income (Loss)	(972.1)	(1,228.5)	(3,290.0)	(3,183.5)
Reclassified out of AOCI	38.3	(26.9)	(28.7)	38.4
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	36.8	(27.7)	(31.7)	37.0
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	50.4	(36.8)	(32.8)	42.2
Tax expense (benefit)	(13.6)	9.1	1.1	(5.2)
Net Income (Loss)	36.8	(27.7)	(31.7)	37.0
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassified out of AOCI	(0.7)	(0.4)	(2.0)	(2.1)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.9)	(0.5)	(2.6)	(2.8)
Tax expense (benefit)	0.2	0.1	0.6	0.7
Net Income (Loss)	(0.7)	(0.4)	(2.0)	(2.1)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(7,703.8)	(8,232.3)	(24,201.1)	(24,723.7)
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	(11.4)	13.2	(2.2)	25.2
Cost of sales and revenues	(3.9)	(9.6)	(28.8)	1.8
Interest and other (income) expenses, net	3.0	(0.9)	1.6	(0.9)
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	3.7	4.2	9.6	1.8
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income) expenses, net	2.5	1.2	5.3	3.6
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income) expenses, net	0.3	0.3	1.0	1.0
Truck, Parts and Other | Settlement loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	2.8	1.5	6.3	4.6
Tax expense (benefit)	(0.6)	(0.3)	(1.3)	(1.1)
Net Income (Loss)	2.2	1.2	5.0	3.5
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	4.6	(1.6)	(0.5)	(3.6)
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	$ 54.4	$ (42.1)	$ (12.5)	$ 17.9