Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (462.2)	$ (819.4)
Investment (income) expense	14.8	19.3
Tax (benefit) expense	102.8	190.4
Net Income (Loss)	(359.4)	(629.0)
Total reclassifications out of AOCI	(44.0)	14.8
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(57.2)	11.1
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(74.9)	14.5
Tax (benefit) expense	17.7	(3.4)
Net Income (Loss)	(57.2)	11.1
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(0.3)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment (income) expense	(0.4)
Tax (benefit) expense	0.1
Net Income (Loss)	(0.3)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(4,778.0)	(6,138.1)
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	0.1	9.4
Cost of sales and revenues	(1.2)	(2.1)
Interest and other (income), net	(6.1)	0.5
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	10.3	4.4
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	0.4	0.4
Truck, Parts and Other | Settlement Loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	7.0
Income before income taxes	17.7	4.8
Tax (benefit) expense	(4.2)	(1.1)
Net Income (Loss)	13.5	3.7
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	$ (67.7)	$ 6.7