Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (669.4)	$ (448.2)
Investment income	10.0	8.1
Tax (benefit) expense	157.3	137.9
Net Income (Loss)	(512.1)	(310.3)
Total reclassifications out of AOCI	16.6	40.8
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	9.3	36.3
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	11.8	50.1
Tax (benefit) expense	(2.5)	(13.8)
Net Income (Loss)	9.3	36.3
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(0.3)	(0.2)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.4)	(0.3)
Tax (benefit) expense	0.1	0.1
Net Income (Loss)	(0.3)	(0.2)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,321.8)	(3,935.7)
Cost of sales and revenues	4,535.5	3,390.9
Interest and other (income), net	(18.7)	(14.0)
Income before income taxes	(591.9)	(383.3)
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	4.6	4.2
Cost of sales and revenues	(1.1)	0.1
Interest and other (income), net	(1.0)
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	7.7	6.7
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	0.4	0.3
Income before income taxes	8.1	7.0
Tax (benefit) expense	(0.5)	(2.3)
Net Income (Loss)	7.6	4.7
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	41.3	34.1
Income before income taxes	(67.5)	(56.8)
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	$ 9.3	$ 45.8