Balance Sheet Classifications, Fair Value, Gross and Pro Forma Net Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Derivative assets	$ 57.8	$ 118.4
Pro forma net amount	48.7	101.9
LIABILITIES
Derivative liabilities	103.0	49.3
Pro forma net amount	93.9	32.8
Interest-Rate Contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(8.7)	(15.4)
LIABILITIES
Less amounts not offset in financial instruments	(8.7)	(15.4)
Foreign-exchange contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments		(0.1)
LIABILITIES
Less amounts not offset in financial instruments		(0.1)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(0.4)	(1.0)
LIABILITIES
Less amounts not offset in financial instruments	(0.4)	(1.0)
Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	0.6	0.3
Designated under hedge accounting
ASSETS
Derivative assets	57.1	113.6
LIABILITIES
Derivative liabilities	100.2	48.2
Designated under hedge accounting | Interest-Rate Contracts | Other Assets
ASSETS
Derivative assets	53.3
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Other Assets
ASSETS
Derivative assets	53.3	109.7
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	98.3	46.3
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	3.8	3.9
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	1.9	1.9
Economic hedge
ASSETS
Derivative assets	0.7	4.8
LIABILITIES
Derivative liabilities	2.8	1.1
Economic hedge | Interest-Rate Contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities		0.1
Economic hedge | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.1	4.0
Economic hedge | Foreign-exchange contracts | Financial Services | Deferred Taxes And Other Liabilities
LIABILITIES
Derivative liabilities	2.2	0.7
Economic hedge | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	$ 0.6	$ 0.8