(Income) Expense Recognized in Earnings Related to Foreign-Exchange Contracts (Detail) - 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]
(Income) Expense recognized in earnings	$ 0.0	$ 0.0	$ 0.0	$ (0.1)
Foreign-exchange contracts
Derivative [Line Items]
(Income) Expense recognized in earnings	(0.8)	4.4	(3.8)	48.7
Truck, Parts and Other | Interest-Rate Contracts | Interest and other (income), net
Derivative [Line Items]
(Income) Expense recognized in earnings	0.0	0.0	0.0	0.0
Truck, Parts and Other | Interest-Rate Contracts | Cost of sales and revenues
Derivative [Line Items]
(Income) Expense recognized in earnings	0.0	0.0	0.0	0.0
Truck, Parts and Other | Foreign-exchange contracts | Interest and other (income), net
Derivative [Line Items]
(Income) Expense recognized in earnings	(0.3)	(1.0)	3.4	3.2
Truck, Parts and Other | Foreign-exchange contracts | Cost of sales and revenues
Derivative [Line Items]
(Income) Expense recognized in earnings	(1.1)	0.6	0.1	1.3
Financial Services | Interest-Rate Contracts | Selling, general and administrative
Derivative [Line Items]
(Income) Expense recognized in earnings	0.0	0.0	0.0	0.0
Financial Services | Interest-Rate Contracts | Interest and other borrowing expenses
Derivative [Line Items]
(Income) Expense recognized in earnings	0.0	0.0	0.0	(0.1)
Financial Services | Foreign-exchange contracts | Selling, general and administrative
Derivative [Line Items]
(Income) Expense recognized in earnings	1.1	0.0	1.6	0.8
Financial Services | Foreign-exchange contracts | Interest and other borrowing expenses
Derivative [Line Items]
(Income) Expense recognized in earnings	$ (0.5)	$ 4.8	$ (8.9)	$ 43.4