Balance Sheet Classifications, Fair Value, Gross and Pro Forma Net Amounts of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
ASSETS
Derivative assets	$ 43.7	$ 20.5
Pro forma net amount	30.6	6.4
LIABILITIES
Derivative liabilities	71.9	159.9
Pro forma net amount	58.8	145.8
Interest-Rate Contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(8.5)	(12.0)
LIABILITIES
Less amounts not offset in financial instruments	(8.5)	(12.0)
Foreign-exchange contracts | Financial Services
ASSETS
Less amounts not offset in financial instruments	(0.5)
LIABILITIES
Less amounts not offset in financial instruments	(0.5)
Foreign-exchange contracts | Truck, Parts and Other
ASSETS
Less amounts not offset in financial instruments	(4.1)	(2.1)
LIABILITIES
Less amounts not offset in financial instruments	(4.1)	(2.1)
Designated under hedge accounting
ASSETS
Derivative assets	43.3	19.1
LIABILITIES
Derivative liabilities	70.3	153.2
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Other Assets
ASSETS
Derivative assets	18.7	16.4
Designated under hedge accounting | Interest-Rate Contracts | Financial Services | Deferred Taxes and Other Liabilities
LIABILITIES
Derivative liabilities	54.3	116.1
Designated under hedge accounting | Foreign-exchange contracts | Financial Services | Other current assets
ASSETS
Derivative assets	1.7
Designated under hedge accounting | Foreign-exchange contracts | Financial Services | Deferred Taxes and Other Liabilities
LIABILITIES
Derivative liabilities	0.5	1.2
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	22.9	2.7
Designated under hedge accounting | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	5.7	35.9
Designated under hedge accounting | Commodity contracts | Truck, Parts and Other | Other current assets
LIABILITIES
Derivative liabilities	9.8
Not designated as hedging instruments
ASSETS
Derivative assets	0.4	1.4
LIABILITIES
Derivative liabilities	1.6	6.7
Not designated as hedging instruments | Foreign-exchange contracts | Financial Services | Other Assets
ASSETS
Derivative assets	0.2	0.3
Not designated as hedging instruments | Foreign-exchange contracts | Financial Services | Deferred Taxes and Other Liabilities
LIABILITIES
Derivative liabilities	0.1	3.4
Not designated as hedging instruments | Foreign-exchange contracts | Truck, Parts and Other | Other current assets
ASSETS
Derivative assets	0.2	1.1
Not designated as hedging instruments | Foreign-exchange contracts | Truck, Parts and Other | Accounts payable, accrued expenses and other
LIABILITIES
Derivative liabilities	$ 1.5	$ 3.3