Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									$ (1,657.9)	$ (3,099.2)	$ (2,810.2)
Investment income									35.9	82.3	60.9
Tax (benefit) expense									359.5	711.3	615.1
Net Income (Loss)	$ (405.8)	$ (385.5)	$ (147.7)	$ (359.4)	$ (531.3)	$ (607.9)	$ (619.7)	$ (629.0)	(1,298.4)	(2,387.9)	(2,195.1)
Total reclassifications out of AOCI									106.3	56.3	(62.6)
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									64.5	39.7	(90.5)
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									87.6	51.7	(121.5)
Tax (benefit) expense									(23.1)	(12.0)	31.0
Net Income (Loss)									64.5	39.7	(90.5)
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									(1.8)	(0.3)	(0.1)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income									(2.4)	(0.4)	(0.2)
Tax (benefit) expense									0.6	0.1	0.1
Net Income (Loss)									(1.8)	(0.3)	(0.1)
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									43.6	16.9	28.0
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,136.0)	(4,538.4)	(2,701.9)	(4,778.0)	(5,710.9)	(6,004.2)	(6,266.5)	(6,138.1)	(17,154.3)	(24,119.7)
Cost of sales and revenues	4,487.0	3,958.3	2,441.5	4,189.6	4,890.0	5,106.8	5,341.7	5,217.1
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									(4.0)	21.2	5.4
Cost of sales and revenues									1.7	(4.1)	(6.6)
Interest and other (income), net									(4.6)	2.1	(1.6)
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net									44.2	20.5	35.3
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net									1.5	1.4	1.4
Truck, Parts and Other | Settlement loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net									11.6
Truck, Parts and Other | Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									57.3	21.9	36.7
Tax (benefit) expense									(13.7)	(5.0)	(8.7)
Net Income (Loss)									43.6	16.9	28.0
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	$ 43.6	$ 45.2	$ 46.9	$ 56.4	$ 57.5	$ 59.6	$ 60.0	$ 53.4
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									$ 94.5	$ 32.5	$ (118.7)