Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	$ 6.4	$ 5.3	$ 12.1	$ 10.4
Income before income taxes	646.2	652.2	216.6	1,213.9
Tax (benefit) expense	(164.9)	(205.0)	(329.9)	(388.3)
Net Income (Loss)	481.3	447.2	(113.3)	825.6
Total reclassification out of AOCI	(4.1)	25.4	37.1	10.5
Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(0.9)	0.7	(4.0)	2.0
Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(7.6)	25.4	50.3	(7.8)
Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	5.0	7.0	9.5	13.8
Pension Plans | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	7.7	10.8	14.5	21.2
Tax (benefit) expense	(2.7)	(3.8)	(5.0)	(7.4)
Net Income (Loss)	5.0	7.0	9.5	13.8
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	(0.3)	(0.2)	(0.8)	(0.8)
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	(0.4)	(0.3)	(1.1)	(1.1)
Tax (benefit) expense	0.1	0.1	0.3	0.3
Net Income (Loss)	(0.3)	(0.2)	(0.8)	(0.8)
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	(8.8)	18.6	28.4	(2.5)
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(8.5)	26.1	46.3	(5.8)
Tax (benefit) expense	(0.3)	(7.5)	(17.9)	3.3
Net Income (Loss)	(8.8)	18.6	28.4	(2.5)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	4,115.8	4,786.1	8,126.4	9,334.1
Cost of sales and revenues	(3,489.4)	(4,061.2)	(6,903.0)	(7,971.4)
Interest and other expense, net	(2.5)	(1.3)	(2.6)	(5.8)
Selling, general and administrative	(110.2)	(108.2)	(220.5)	(217.7)
Income before income taxes	562.5	556.1	46.9	1,023.7
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	3.5	5.8	6.9	11.2
Selling, general and administrative	3.6	4.2	6.5	8.5
Income before income taxes	7.1	10.0	13.4	19.7
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	(1.7)	0.3	(6.5)	0.3
Cost of sales and revenues	0.6	0.2	0.6	3.4
Interest and other expense, net	0.2	0.2	1.9	(1.7)
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	(25.1)	(23.9)	(49.6)	(47.4)
Interest and other borrowing expenses	(32.6)	(29.6)	(62.9)	(58.7)
Income before income taxes	77.3	90.8	157.6	179.8
Financial Services | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	0.3	0.5	0.5	0.9
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	$ (7.6)	$ 25.4	$ 50.3	$ (7.8)