Balance Sheet Classifications, Fair Value, Gross and Pro-Forma Net Amounts of Derivative Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
ASSETS
Derivative assets	$ 64.2	$ 48.0
Pro-forma net amount	60.6	31.7
LIABILITIES
Derivative liabilities	49.6	68.6
Pro-forma net amount	46.0	52.3
Interest Rate Contract | Financial Services
ASSETS
Less amounts not offset in financial instruments	(3.5)	(16.1)
LIABILITIES
Less amounts not offset in financial instruments	(3.5)	(16.1)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(0.1)	(0.2)
LIABILITIES
Less amounts not offset in financial instruments	(0.1)	(0.2)
Designated as Hedging Instrument
ASSETS
Derivative assets	56.2	46.3
LIABILITIES
Derivative liabilities	48.8	68.3
Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Other Assets
ASSETS
Derivative assets	54.5	46.3
Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	48.2	67.7
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	1.7
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Accounts Payable Accrued Expenses And Other
LIABILITIES
Derivative liabilities	0.6	0.6
Not Designated as Hedging Instrument
ASSETS
Derivative assets	8.0	1.7
LIABILITIES
Derivative liabilities	0.8	0.3
Not Designated as Hedging Instrument | Interest Rate Contract | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	0.2
Not Designated as Hedging Instrument | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	7.3	1.1
Not Designated as Hedging Instrument | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	0.1	0.1
Not Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	0.7	0.6
Not Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Accounts Payable Accrued Expenses And Other
LIABILITIES
Derivative liabilities	$ 0.5	$ 0.2