Expense (Income) Recognized in Earnings Related to Foreign-Exchange Contracts (Detail) - Foreign-exchange contracts - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
(Income) Expense recognized in earnings	$ 6.3	$ 70.1
Truck, Parts and Other | Interest and other (income), net
Derivative [Line Items]
(Income) Expense recognized in earnings	3.4	31.5
Truck, Parts and Other | Cost of sales and revenues
Derivative [Line Items]
(Income) Expense recognized in earnings	1.2	3.3
Financial Services | Interest and other borrowing expenses
Derivative [Line Items]
(Income) Expense recognized in earnings	0.5	39.2
Financial Services | Selling, general and administrative
Derivative [Line Items]
(Income) Expense recognized in earnings	$ 1.2	$ (3.9)