Balance Sheet Classifications and Fair Value of Derivative Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
ASSETS
Derivative assets	$ 13.4	$ 5.1
LIABILITIES
Derivative liabilities	93.0	113.0
Interest Rate Contract | Financial Services
ASSETS
Derivative assets	12.7	4.6
LIABILITIES
Derivative liabilities	91.6	112.3
Foreign-exchange contracts | Financial Services
ASSETS
Derivative assets	0.3
LIABILITIES
Derivative liabilities		0.4
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Derivative assets	0.4	0.5
LIABILITIES
Derivative liabilities	1.4	0.3
Designated as Hedging Instrument
ASSETS
Derivative assets	12.2	4.8
LIABILITIES
Derivative liabilities	92.7	111.8
Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Other Assets
ASSETS
Derivative assets	12.0	4.6
Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	91.6	111.7
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Deferred taxes and other current assets
ASSETS
Derivative assets	0.2	0.2
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Accounts Payable and Accrued Liabilities
LIABILITIES
Derivative liabilities	1.1	0.1
Not Designated as Hedging Instrument
ASSETS
Derivative assets	1.2	0.3
LIABILITIES
Derivative liabilities	0.3	1.2
Not Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Other Assets
ASSETS
Derivative assets	0.7
Not Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities		0.6
Not Designated as Hedging Instrument | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.3
Not Designated as Hedging Instrument | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities		0.4
Not Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Deferred taxes and other current assets
ASSETS
Derivative assets	0.2	0.3
Not Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Accounts Payable and Accrued Liabilities
LIABILITIES
Derivative liabilities	$ 0.3	$ 0.2