Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest-Rate Contracts
Derivative [Line Items]
(Loss) gain recognized in OCI	$ (108.1)	$ (30.9)	$ 33.8
Foreign-exchange contracts
Derivative [Line Items]
(Loss) gain recognized in OCI	(17.4)	24.4	4.9
Truck, Parts and Other | Foreign-exchange contracts
Derivative [Line Items]
(Loss) gain recognized in OCI	(17.4)	24.4	4.9
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
(Loss) gain recognized in OCI	$ (108.1)	$ (30.9)	$ 33.8