Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	$ 5.8	$ 6.5
Pre-tax expense reduction	406.5	337.2
Tax expense (benefit)	(132.6)	(101.1)
Net Income	273.9	236.1
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net Income	(1.0)	(5.5)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and (Losses) on Derivative Contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax expense reduction	(6.9)	(18.1)
Tax expense (benefit)	1.9	5.3
Net Income	(5.0)	(12.8)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains on Marketable Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Investment income	0.4
Tax expense (benefit)	(0.1)
Net Income	0.3
Reclassification out of Accumulated Other Comprehensive Income | Pension Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax expense reduction	5.5	11.2
Tax expense (benefit)	(1.8)	(3.9)
Net Income	3.7	7.3
Truck, Parts and Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	(3,595.5)	(3,189.3)
Interest and other expense, net	(1.4)	(1.5)
Pre-tax expense reduction	315.2	250.6
Truck, Parts and Other | Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and (Losses) on Derivative Contracts | Foreign-exchange contracts
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of sales and revenues	0.1	(2.3)
Interest and other expense, net	(0.1)	(0.2)
Truck, Parts and Other | Reclassification out of Accumulated Other Comprehensive Income | Pension Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	0.3
Actuarial loss	4.9	10.5
Truck, Parts and Other | Reclassification out of Accumulated Other Comprehensive Income | Pension Plan | Cost of sales and revenues
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	0.2	0.3
Actuarial loss	2.7	5.7
Truck, Parts and Other | Reclassification out of Accumulated Other Comprehensive Income | Pension Plan | SG&A
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	0.1
Actuarial loss	2.2	4.8
Financial Services
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	(36.6)	(38.9)
Pre-tax expense reduction	85.5	80.1
Financial Services | Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains and (Losses) on Derivative Contracts | Interest Rate Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest and other borrowing expenses	(6.9)	(15.6)
Financial Services | Reclassification out of Accumulated Other Comprehensive Income | Pension Plan | SG&A
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial loss	$ 0.3	$ 0.4