Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest-Rate Contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	$ 2.9	$ (23.2)	$ 84.5	$ (106.3)
Foreign-exchange contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	(11.3)	(3.0)	(4.9)	(23.4)
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	(11.3)	(3.0)	(4.9)	(23.4)
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	2.9	(23.2)	84.5	(106.3)
Financial Services | Foreign-exchange contracts
Derivative [Line Items]
Gain (Loss) recognized in OCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0