Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ (38.1)	$ 8.4	$ (83.1)	$ (53.3)
Interest-Rate Contracts | Truck, Parts and Other
Derivative [Line Items]
Gain (loss) recognized in OCI	0.0	0.0	0.0	0.0
Interest-Rate Contracts | Financial Services
Derivative [Line Items]
Gain (loss) recognized in OCI	(38.1)	8.4	(83.1)	(53.3)
Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	(1.7)	9.6	(20.4)	9.9
Foreign-exchange contracts | Truck, Parts and Other
Derivative [Line Items]
Gain (loss) recognized in OCI	(1.7)	9.6	(20.4)	9.9
Foreign-exchange contracts | Financial Services
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0