Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									$ (2,173.3)	$ (1,130.4)	$ (2,337.1)
Tax (benefit) expense									498.1	608.7	733.1
Net Income (Loss)	$ (589.2)	$ (402.7)	$ (373.0)	$ (310.3)	$ (288.8)	$ (346.2)	$ (481.3)	$ 594.6	(1,675.2)	(521.7)	(1,604.0)
Investment income									(35.3)	(27.6)	(21.8)
Total reclassifications out of AOCI									114.8	18.1	5.3
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									97.1	1.9	(20.8)
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									133.4	10.8	(29.3)
Tax (benefit) expense									(36.3)	(8.9)	8.5
Net Income (Loss)									97.1	1.9	(20.8)
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									(0.4)	(2.7)	(1.5)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax (benefit) expense									0.2	1.0	0.6
Net Income (Loss)									(0.4)	(2.7)	(1.5)
Investment income									(0.6)	(3.7)	(2.1)
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI									18.1	18.9	27.6
Pension Plans | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									26.6	28.9	42.4
Tax (benefit) expense									(8.5)	(10.0)	(14.8)
Net Income (Loss)									18.1	18.9	27.6
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(5,122.4)	(4,731.5)	(4,397.9)	(3,935.7)	(3,767.0)	(3,953.2)	(4,115.8)	(4,010.6)	(18,187.5)	(15,846.6)	(17,942.8)
Cost of sales and revenues	4,409.5	4,046.8	3,755.2	3,382.2	3,243.2	3,371.5	3,489.4	3,413.6	15,593.7	13,517.7	15,292.1
Interest and other expense, net									5.6	11.6	12.3
Income before income taxes									(1,874.0)	(796.3)	(1,952.7)
Selling, general and administrative									449.5	440.8	445.9
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									12.1	(27.9)	(0.1)
Cost of sales and revenues									3.9	0.6	3.4
Interest and other expense, net									1.8	1.3	(4.1)
Truck, Parts and Other | Derivative Contracts | Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues									0.0	0.0	0.0
Cost of sales and revenues									0.0	0.0	0.0
Interest and other expense, net									0.0	0.0	0.0
Truck, Parts and Other | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues									12.3	13.6	22.4
Income before income taxes									24.6	26.6	39.5
Selling, general and administrative									12.3	13.0	17.1
Truck, Parts and Other | Prior service costs | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues									1.0	0.9	1.0
Income before income taxes									1.2	1.2	1.2
Selling, general and administrative									0.2	0.3	0.2
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	$ 39.8	$ 38.3	$ 37.4	$ 34.1	$ 32.1	$ 32.2	$ 32.6	$ 30.3	149.6	127.2	118.0
Income before income taxes									(264.0)	(306.5)	(362.6)
Selling, general and administrative									105.5	99.4	95.6
Financial Services | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									17.8	(26.0)	(0.8)
Financial Services | Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes									115.6	36.8	(28.5)
Financial Services | Derivative Contracts | Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses									0.0	0.0	0.0
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									115.6	36.8	(28.5)
Financial Services | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative									$ 0.8	$ 1.1	$ 1.7