Balance Sheet Classifications, Fair Value, Gross and Pro Forma Net Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Derivative assets	$ 50.0	$ 57.8
Pro forma net amount	38.4	48.7
LIABILITIES
Derivative liabilities	112.3	103.0
Pro forma net amount	100.7	93.9
Interest-Rate Contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(11.0)	(8.7)
LIABILITIES
Less amounts not offset in financial instruments	(11.0)	(8.7)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(0.6)	(0.4)
LIABILITIES
Less amounts not offset in financial instruments	(0.6)	(0.4)
Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	0.4	0.6
Designated under hedge accounting
ASSETS
Derivative assets	49.1	57.1
LIABILITIES
Derivative liabilities	111.0	100.2
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Other Assets
ASSETS
Derivative assets	45.7	53.3
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	108.2	98.3
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	3.4	3.8
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	2.8	1.9
Not designated as hedging instruments
ASSETS
Derivative assets	0.9	0.7
LIABILITIES
Derivative liabilities	1.3	2.8
Not designated as hedging instruments | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.7	0.1
Not designated as hedging instruments | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	0.9	2.2
Not designated as hedging instruments | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	$ 0.2	$ 0.6