Balance Sheet Classifications, Fair Value, Gross and Pro Forma Net Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
ASSETS
Derivative assets	$ 94.3	$ 137.3
Pro forma net amount	85.6	133.4
LIABILITIES
Derivative liabilities	62.7	48.2
Pro forma net amount	54.0	44.3
Interest-Rate Contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(7.8)	(3.3)
LIABILITIES
Less amounts not offset in financial instruments	(7.8)	(3.3)
Foreign-exchange contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(0.1)	(0.2)
LIABILITIES
Less amounts not offset in financial instruments	(0.1)	(0.2)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(0.8)	(0.4)
LIABILITIES
Less amounts not offset in financial instruments	(0.8)	(0.4)
Designated under hedge accounting
ASSETS
Derivative assets	93.8	136.1
LIABILITIES
Derivative liabilities	58.5	46.9
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	54.8	46.7
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Other Assets
ASSETS
Derivative assets	90.3	132.2
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	3.7	0.2
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	3.5	3.9
Economic hedge
ASSETS
Derivative assets	0.5	1.2
LIABILITIES
Derivative liabilities	4.2	1.3
Economic hedge | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	2.9	1.0
Economic hedge | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.2	0.3
Economic hedge | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	1.3	0.3
Economic hedge | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	$ 0.3	$ 0.9