Gross and Proforma Net Amounts In Statement of Financial Position of Derivative Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	$ 13.4	$ 5.1
Assets,Amounts Not Offset in Financial Instruments	(6.0)	(2.6)
Assets, Pro Forma Net Amount	7.4	2.5
Liabilities, Amount Recognized in Balance Sheet	93.0	113.0
Liabilities, Amounts Not Offset in Financial Instruments	(6.0)	(2.6)
Liabilities, Pro Forma Net Amount	87.0	110.4
Truck, Parts and Other | Foreign-exchange contracts
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	0.4	0.5
Assets, Pro Forma Net Amount	0.4	0.5
Liabilities, Amount Recognized in Balance Sheet	1.4	0.3
Liabilities, Pro Forma Net Amount	1.4	0.3
Financial Services | Foreign-exchange contracts
Derivative Instruments [Line Items]
Assets, Amount Recognized in Balance Sheet	0.3
Assets, Pro Forma Net Amount	0.3
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	12.7	4.6
Assets,Amounts Not Offset in Financial Instruments	(6.0)	(2.6)
Assets, Pro Forma Net Amount	6.7	2.0
Liabilities, Amount Recognized in Balance Sheet	91.6	112.3
Liabilities, Amounts Not Offset in Financial Instruments	(6.0)	(2.6)
Liabilities, Pro Forma Net Amount	$ 85.6	$ 109.7