Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 8.4	$ (19.9)	$ (53.3)	$ 11.8
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	8.4	(19.9)	(53.3)	11.8
Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	9.6	(3.0)	9.9	(3.0)
Foreign-exchange contracts | Truck, Parts and Other
Derivative [Line Items]
Gain (loss) recognized in OCI	9.6	(3.0)	9.9	(3.0)
Foreign-exchange contracts | Financial Services
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0