Balance Sheet Classification of Gross and Proforma Net Amounts of Derivative Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	$ 30.9	$ 5.1
Assets,Amounts Not Offset in Financial Instruments	(12.4)	(2.6)
Assets, Pro Forma Net Amount	18.5	2.5
Liabilities, Amount Recognized in Balance Sheet	75.5	113.0
Liabilities, Amounts Not Offset in Financial Instruments	(12.4)	(2.6)
Liabilities, Pro Forma Net Amount	63.1	110.4
Foreign-exchange contracts
Derivative Instruments [Line Items]
Liabilities, Amount Recognized in Balance Sheet	1.4
Liabilities, Pro Forma Net Amount	1.4
Truck, Parts and Other | Foreign-exchange contracts
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	0.4	0.5
Assets,Amounts Not Offset in Financial Instruments	(0.1)
Assets, Pro Forma Net Amount	0.3	0.5
Liabilities, Amount Recognized in Balance Sheet	3.1	0.3
Liabilities, Amounts Not Offset in Financial Instruments	(0.1)
Liabilities, Pro Forma Net Amount	3.0	0.3
Financial Services | Foreign-exchange contracts
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	0.2
Assets, Pro Forma Net Amount	0.2
Liabilities, Amount Recognized in Balance Sheet		0.4
Liabilities, Pro Forma Net Amount		0.4
Financial Services | Interest Rate Contract
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	30.3	4.6
Assets,Amounts Not Offset in Financial Instruments	(12.3)	(2.6)
Assets, Pro Forma Net Amount	18.0	2.0
Liabilities, Amount Recognized in Balance Sheet	71.0	112.3
Liabilities, Amounts Not Offset in Financial Instruments	(12.3)	(2.6)
Liabilities, Pro Forma Net Amount	$ 58.7	$ 109.7