Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	$ (23.5)	$ (35.6)	$ 22.7
Unrealized (gains) and losses on derivative contracts, tax	5.1	10.8	(7.8)
Unrealized (gains) and losses on derivative contracts, after-tax	(18.4)	(24.8)
Unrealized (gains) and losses on marketable debt securities, pre-tax	(0.9)	1.7	(2.9)
Unrealized (gains) and losses on marketable debt securities, tax	0.3	(0.5)	0.8
Unrealized (gains) and losses on marketable debt securities, after tax	(0.6)	1.2
Pension plans, pre- tax	22.0	45.3	45.4
Pension plans, tax	(7.1)	(15.8)	(15.2)
Pension plans, after tax	14.9	29.5
Foreign currency translation, after tax		(2.0)
Total reclassifications out of AOCI	(4.1)	3.9
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	20.1	41.7
Pension plans, Prior service costs, pre-tax	1.2	1.3
Truck, Parts and Other | Cost of sales and revenues
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	11.1	21.4
Pension plans, Prior service costs, pre-tax	1.0	0.4
Truck, Parts and Other | Interest and other expense (income), net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation, after tax		(1.1)
Truck, Parts and Other | SG&A
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pension plans, Actuarial loss, pre- tax	9.0	20.3
Pension plans, Prior service costs, pre-tax	0.2	0.6
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	0.3	1.0
Truck, Parts and Other | Foreign-exchange contracts | Interest and other expense (income), net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized (gains) and losses on derivative contracts, pre-tax	(2.1)	(0.6)
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	0.7	2.3
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	$ (21.7)	$ (36.0)