Pre-Tax Effects of Gain (Loss) on Cash Flow Hedges Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 43.4	$ (49.1)	$ 67.1	$ (27.3)
Commodity contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	(9.8)		(9.8)
Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	2.9	(10.8)	(9.0)	42.6
Truck, Parts and Other | Commodity contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	(9.8)		(9.8)
Truck, Parts and Other | Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	2.3	(10.8)	(6.7)	42.6
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	43.4	$ (49.1)	67.1	$ (27.3)
Financial Services | Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 0.6		$ (2.3)