Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (819.4)	$ (669.4)
Investment income	19.3	10.0
Tax (benefit) expense	190.4	157.3
Net Income (Loss)	(629.0)	(512.1)
Total reclassifications out of AOCI	14.8	15.2
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	11.1	9.3
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	14.5	11.8
Tax (benefit) expense	(3.4)	(2.5)
Net Income (Loss)	11.1	9.3
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		(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	(6,138.1)	(5,321.8)
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	9.4	4.6
Cost of sales and revenues	(2.1)	(1.1)
Interest and other (income), net	0.5	(1.0)
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	4.4	7.7
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	0.4	0.4
Income before income taxes	4.8	8.1
Tax (benefit) expense	(1.1)	(1.9)
Net Income (Loss)	3.7	6.2
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	$ 6.7	$ 9.3