Amount of (Gain) Loss from Derivative Financial Instruments Reclassified from AOCI into Consolidated Statements of Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Truck, Parts and Other
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	$ (12.5)	$ 14.6	$ (2.8)
Financial Services
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	95.6	34.0	(116.9)
Fair Value Hedging | Financial Services
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	1.1	1.5	1.8
Cash Flow Hedging | Truck, Parts and Other
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	(6.9)	19.2	(2.8)
Cash Flow Hedging | Financial Services
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	94.5	32.5	$ (118.7)
Net Investment Hedging | Truck, Parts and Other
Derivative Instruments Gain Loss [Line Items]
Derivatives designated as hedging instruments (gain) loss	$ (5.6)	$ (4.6)