Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	$ 9.8	$ (29.8)
Unrealized (gains) and losses on derivative contracts, tax	(4.1)	9.0
Unrealized (gains) and losses on derivative contracts, after-tax	5.7	(20.8)
Unrealized (gains) and losses on marketable debt securities, pre-tax	(0.8)	1.4
Unrealized (gains) and losses on marketable debt securities, tax	0.3	(0.4)
Unrealized (gains) and losses on marketable debt securities, after tax	(0.5)	1.0
Pension plans, pre- tax	16.6	33.9
Pension plans, tax	(5.4)	(11.8)
Pension plans, after tax	11.2	22.1
Foreign currency translation, after tax		(2.0)
Total reclassifications out of AOCI	16.4	0.3
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	14.9	31.7
Pension plans, Prior service costs, pre-tax	0.9	0.9
Foreign currency translation, after tax		(1.1)
Truck, Parts and Other | Cost of sales and revenues
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	8.4	17.4
Pension plans, Prior service costs, pre-tax	0.7	0.8
Truck, Parts and Other | SG&A
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	6.5	14.2
Pension plans, Prior service costs, pre-tax	0.2	0.1
Truck, Parts and Other | Foreign-exchange contracts | Cost of sales and revenues
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	1.7	(0.5)
Truck, Parts and Other | Foreign-exchange contracts | Interest and other expense, net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	0.1	(0.2)
Financial Services | Interest Expense and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation, after tax		(0.9)
Financial Services | SG&A
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	0.8	1.3
Financial Services | Interest Rate Contract | Interest Expense and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	$ 8.0	$ (29.1)