Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	$ (8.7)	$ (6.4)	$ (16.8)	$ (12.1)
Income before income taxes	(539.0)	(646.2)	(987.2)	(216.6)
Tax (benefit) expense	166.0	164.9	303.9	329.9
Net Income (Loss)	(373.0)	(481.3)	(683.3)	113.3
Total reclassifications out of AOCI	40.9	(4.1)	81.7	37.1
Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	11.4	(0.9)	15.7	(4.0)
Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	40.0	(7.6)	85.8	50.3
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	4.3	5.0	9.0	9.5
Pension Plans | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	6.2	7.7	13.2	14.5
Tax (benefit) expense	(1.9)	(2.7)	(4.2)	(5.0)
Net Income (Loss)	4.3	5.0	9.0	9.5
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	(0.1)	(0.3)	(0.3)	(0.8)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.1)	(0.4)	(0.4)	(1.1)
Tax (benefit) expense		0.1	0.1	0.3
Net Income (Loss)	(0.1)	(0.3)	(0.3)	(0.8)
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications out of AOCI	36.7	(8.8)	73.0	28.4
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	51.4	(8.5)	101.5	46.3
Tax (benefit) expense	(14.7)	(0.3)	(28.5)	(17.9)
Net Income (Loss)	36.7	(8.8)	73.0	28.4
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	(4,397.9)	(4,115.8)	(8,333.6)	(8,126.4)
Cost of sales and revenues	3,755.2	3,489.4	7,137.4	6,903.0
Interest and other expense (income), net	1.5	2.5	(0.1)	2.6
Selling, general and administrative	107.8	110.2	219.1	220.5
Income before income taxes	(467.3)	(562.5)	(850.1)	(46.9)
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	0.3	0.3	0.5	0.5
Selling, general and administrative			0.1	0.1
Income before income taxes			0.6	0.6
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	2.7	3.5	6.1	6.9
Selling, general and administrative	3.0	3.6	6.1	6.5
Income before income taxes	5.7	7.1	12.2	13.4
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	(1.7)	16.3	(6.5)
Cost of sales and revenues	(1.3)	0.6	(1.2)	0.6
Interest and other expense (income), net	0.6	0.2	0.6	1.9
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	0.0
Cost of sales and revenues	0.0	0.0	0.0	0.0
Interest and other expense (income), net	0.0	0.0	0.0	0.0
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	26.3	25.1	51.5	49.6
Interest and other borrowing expenses	37.4	32.6	71.5	62.9
Income before income taxes	(63.0)	(77.3)	(120.3)	(157.6)
Financial Services | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	0.2	0.3	0.4	0.5
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	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	$ 40.0	$ (7.6)	$ 85.8	$ 50.3