Pre-Tax Effects of Derivative Instruments Recognized in OCI (Detail) - Derivative Contracts - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ (23.2)	$ (25.6)	$ (106.3)	$ (78.9)
Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	(3.0)	13.9	(23.4)	23.8
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	(3.0)	13.9	(23.4)	23.8
Financial Services | Interest-Rate Contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	(23.2)	(25.6)	(106.3)	(78.9)
Financial Services | Foreign-exchange contracts
Derivative [Line Items]
Gain (loss) recognized in OCI	$ 0.0	$ 0.0	$ 0.0	$ 0.0