Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest-Rate Contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	$ 93.1	$ (38.1)	$ 81.6	$ (83.1)
Foreign-exchange contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	10.8	(1.7)	6.4	(20.4)
Truck, Parts and Other | Interest-Rate Contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	0.0	0.0	0.0	0.0
Truck, Parts and Other | Foreign-exchange contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	10.8	(1.7)	6.4	(20.4)
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	93.1	(38.1)	81.6	(83.1)
Financial Services | Foreign-exchange contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0