Expense (Income) Reclassified out of AOCI into Income (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
Derivative [Line Items]
Income before income taxes									$ (2,173.3)	$ (1,130.4)	$ (2,337.1)
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Income before income taxes									133.4	10.8	(29.3)
Truck, Parts and Other
Derivative [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)
Financial Services
Derivative [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)
Interest-Rate Contracts | Truck, Parts and Other | Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Derivative [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
Interest-Rate Contracts | Financial Services | Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Income before income taxes									115.6	36.8	(28.5)
Interest-Rate Contracts | Financial Services | Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest and other borrowing expenses									115.6	36.8	(28.5)
Foreign-exchange contracts | Truck, Parts and Other | Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Derivative [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)
Foreign-exchange contracts | Financial Services | Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Income before income taxes									17.8	(26.0)	(0.8)
Foreign-exchange contracts | Financial Services | Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Derivative [Line Items]
Interest and other borrowing expenses									$ 0.0	$ 0.0	$ 0.0