Amount of Expense (Income) from Derivative Financial Instruments Recognized in Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments expense (income)	$ 11.6	$ (96.6)	$ 26.7	$ (84.4)
Fair Value Hedging | Financial Services
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments expense (income)	0.3	0.5	0.9	0.9
Cash Flow Hedging | Truck, Parts and Other
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments expense (income)	6.3	(5.0)	14.1	(2.5)
Cash Flow Hedging | Financial Services
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments expense (income)	$ 5.0	$ (92.1)	$ 11.7	$ (82.8)