Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									$ (3,099.2)	$ (2,810.2)	$ (2,173.3)
Investment income									82.3	60.9	35.3
Tax (benefit) expense									711.3	615.1	498.1
Net Income (Loss)	$ (531.3)	$ (607.9)	$ (619.7)	$ (629.0)	$ (578.1)	$ (545.3)	$ (559.6)	$ (512.1)	(2,387.9)	(2,195.1)	(1,675.2)
Total reclassifications out of AOCI									56.3	(62.6)	114.8
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									39.7	(90.5)	97.1
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									51.7	(121.5)	133.4
Tax (benefit) expense									(12.0)	31.0	(36.3)
Net Income (Loss)									39.7	(90.5)	97.1
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									(0.3)	(0.1)	(0.4)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income									(0.4)	(0.2)	(0.6)
Tax (benefit) expense									0.1	0.1	0.2
Net Income (Loss)									(0.3)	(0.1)	(0.4)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,710.9)	(6,004.2)	(6,266.5)	(6,138.1)	(5,932.7)	(5,416.9)	(5,467.2)	(5,321.8)	(24,119.7)	(22,138.6)
Cost of sales and revenues	4,890.0	5,106.8	5,341.7	5,217.1	5,088.6	4,653.6	4,647.3	4,535.5
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									21.2	5.4	12.1
Cost of sales and revenues									(4.1)	(6.6)	3.9
Interest and other (income), net									2.1	(1.6)	1.8
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net									20.5	35.3	25.4
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net									1.4	1.4	1.2
Income before income taxes									21.9	36.7	26.6
Tax (benefit) expense									(5.0)	(8.7)	(8.5)
Net Income (Loss)									16.9	28.0	18.1
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	$ 57.5	$ 59.6	$ 60.0	$ 53.4	$ 50.9	$ 49.0	$ 45.7	$ 41.3
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									$ 32.5	$ (118.7)	$ 115.6