Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (719.6)	$ (539.0)	$ (1,389.0)	$ (987.2)
Investment income	14.6	8.7	24.6	16.8
Tax (benefit) expense	160.0	166.0	317.3	303.9
Net Income (Loss)	(559.6)	(373.0)	(1,071.7)	(683.3)
Total reclassifications out of AOCI	(64.9)	40.9	(49.7)	81.7
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(72.8)	36.7	(63.5)	73.0
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(97.1)	51.4	(85.3)	101.5
Tax (benefit) expense	24.3	(14.7)	21.8	(28.5)
Net Income (Loss)	(72.8)	36.7	(63.5)	73.0
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	0.0	(0.1)	(0.3)	(0.3)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	0.0	(0.1)	(0.4)	(0.4)
Tax (benefit) expense	0.0	0.0	0.1	0.1
Net Income (Loss)	0.0	(0.1)	(0.3)	(0.3)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,467.2)		(10,789.0)
Cost of sales and revenues	4,647.3		9,182.8
Income before income taxes	(632.6)		(1,224.5)
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	(2.8)	12.1	1.8	16.3
Cost of sales and revenues	(2.5)	(1.3)	(3.6)	(1.2)
Interest and other (income), net	0.3	0.6	(0.7)	0.6
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	10.1	5.9	17.8	12.6
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	0.3	0.3	0.7	0.6
Income before income taxes	10.4	6.2	18.5	13.2
Tax (benefit) expense	(2.5)	(1.9)	(4.4)	(4.2)
Net Income (Loss)	7.9	4.3	14.1	9.0
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	$ (92.1)	$ 40.0	$ (82.8)	$ 85.8