Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ (668.8)	$ (581.8)	$ (2,057.8)	$ (1,569.0)
Investment income	16.4	9.0	41.0	25.8
Tax (benefit) expense	123.5	179.1	440.8	483.0
Net Income (Loss)	(545.3)	(402.7)	(1,617.0)	(1,086.0)
Total reclassifications out of AOCI	5.9	25.4	(43.8)	107.1
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(1.0)	21.0	(64.5)	94.0
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(0.5)	28.1	(85.8)	129.6
Tax (benefit) expense	(0.5)	(7.1)	21.3	(35.6)
Net Income (Loss)	(1.0)	21.0	(64.5)	94.0
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	0.1	(0.1)	(0.2)	(0.4)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	0.1	(0.2)	(0.3)	(0.6)
Tax (benefit) expense	0.0	0.1	0.1	0.2
Net Income (Loss)	0.1	(0.1)	(0.2)	(0.4)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,416.9)		(16,205.9)
Cost of sales and revenues	4,653.6		13,836.4
Income before income taxes	(573.6)		(1,798.1)
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	4.3	(3.0)	6.1	13.3
Cost of sales and revenues	(1.6)	5.5	(5.2)	4.3
Interest and other (income), net	0.5	1.0	(0.2)	1.6
Truck, Parts and Other | Actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	8.7	6.3	26.5	18.9
Truck, Parts and Other | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other (income), net	0.3	0.3	1.0	0.9
Income before income taxes	9.0	6.6	27.5	19.8
Tax (benefit) expense	(2.2)	(2.1)	(6.6)	(6.3)
Net Income (Loss)	6.8	4.5	20.9	13.5
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	$ (3.7)	$ 24.6	$ (86.5)	$ 110.4