Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest-Rate Contracts
Derivative [Line Items]
Loss recognized in OCI	$ (11.5)	$ (45.0)
Foreign-exchange contracts
Derivative [Line Items]
Loss recognized in OCI	(4.4)	(18.7)
Truck, Parts and Other | Interest-Rate Contracts
Derivative [Line Items]
Loss recognized in OCI	0.0	0.0
Truck, Parts and Other | Foreign-exchange contracts
Derivative [Line Items]
Loss recognized in OCI	(4.4)	(18.7)
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
Loss recognized in OCI	(11.5)	(45.0)
Financial Services | Foreign-exchange contracts
Derivative [Line Items]
Loss recognized in OCI	$ 0.0	$ 0.0