Balance Sheet Classifications, Fair Value, Gross and Pro-Forma Net Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Derivative assets	$ 137.3	$ 89.2
Pro-forma net amount	133.4	84.4
LIABILITIES
Derivative liabilities	48.2	48.5
Pro-forma net amount	44.3	43.7
Interest-Rate Contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(3.3)	(3.9)
LIABILITIES
Less amounts not offset in financial instruments	(3.3)	(3.9)
Foreign-exchange contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(0.2)
LIABILITIES
Less amounts not offset in financial instruments	(0.2)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(0.4)	(0.9)
LIABILITIES
Less amounts not offset in financial instruments	(0.4)	(0.9)
Designated as Hedging Instrument
ASSETS
Derivative assets	136.1	83.9
LIABILITIES
Derivative liabilities	46.9	47.6
Designated as Hedging Instrument | Interest-Rate Contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	46.7	45.7
Designated as Hedging Instrument | Interest-Rate Contracts | Financial Services | Other Assets
ASSETS
Derivative assets	132.2	82.7
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	0.2	1.9
Designated as Hedging Instrument | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	3.9	1.2
Economic hedge
ASSETS
Derivative assets	1.2	5.3
LIABILITIES
Derivative liabilities	1.3	0.9
Economic hedge | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	1.0
Economic hedge | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.3	3.4
Economic hedge | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	0.3	0.9
Economic hedge | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	$ 0.9	$ 1.9