Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	$ 429.6	$ (561.7)
Tax (benefit) expense	165.0	183.3
After-tax expense (income)	594.6	(378.4)
Total reclassification out of AOCI	41.2	(14.9)
Investment income	(5.7)	(5.1)
Foreign-exchange contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	3.1	(1.3)
Interest-Rate Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	(57.9)	33.2
Pension Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	(4.5)	(6.8)
Pension Plan | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	6.8	10.4
Tax (benefit) expense	(2.3)	(3.6)
After-tax expense (income)	4.5	6.8
Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	0.5	0.6
Marketable Debt Securities | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax (benefit) expense	0.2	0.2
After-tax expense (income)	(0.5)	(0.6)
Investment income	(0.7)	(0.8)
Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassification out of AOCI	(37.2)	21.1
Derivative Contracts | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income before income taxes	54.8	(31.9)
Tax (benefit) expense	(17.6)	10.8
After-tax expense (income)	37.2	(21.1)
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net sales and revenues	4,010.6	4,548.0
Cost of sales and revenues	3,413.6	3,910.2
Interest and other expense, net	0.1	4.5
Selling, general and administrative	110.3	109.5
Income before income taxes	515.6	(467.6)
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	0.2	0.2
Selling, general and administrative	0.1	0.1
Income before income taxes	0.3	0.3
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	3.4	5.4
Selling, general and administrative	2.9	4.3
Income before income taxes	6.3	9.7
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	0.5
Cost of sales and revenues	(5.3)	3.2
Interest and other expense, net	1.7	(1.9)
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	24.5	23.5
Interest and other borrowing expenses	30.3	29.1
Income before income taxes	(80.3)	(89.0)
Financial Services | Actuarial loss | Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Selling, general and administrative	0.2	0.4
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	$ 57.9	$ (33.2)