Balance Sheet Classifications of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Deferred Tax Liabilities	$ (570.0)	$ (590.5)
Truck, Parts and Other | Other Noncurrent Assets
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Deferred Tax Assets Net	90.4	115.4
Truck, Parts and Other | Other liabilities
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Deferred Tax Liabilities	(3.5)	(5.3)
Financial Services | Other Assets
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Deferred Tax Assets Net	46.8	36.8
Financial Services | Deferred Taxes and Other Liabilities
Schedule Of Deferred Income Tax Assets And Liabilities [Line Items]
Deferred Tax Liabilities	$ (703.7)	$ (737.4)