Amortized Cost Basis of Impaired Loans and Specific Reserve (Detail) - Financial Services - USD ($) $ in Millions		9 Months Ended		12 Months Ended
		Sep. 30, 2021		Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Impaired loans with a specific reserve		$ 16.2		$ 24.7
Associated allowance		(0.8)		(1.9)
Net carrying amount of impaired loans with specific reserve		15.4		22.8
Impaired loans with no specific reserve		7.3		1.2
Net carrying amount of impaired loans		22.7		24.0
Average amortized cost basis		26.8	[1]	34.3	[2]
Dealer | Wholesale
Financing Receivable, Impaired [Line Items]
Average amortized cost basis	[2]			4.9
Dealer | Retail
Financing Receivable, Impaired [Line Items]
Impaired loans with no specific reserve		1.2		1.2
Net carrying amount of impaired loans		1.2		1.2
Average amortized cost basis		1.2	[1]	1.9	[2]
Customer Retail | Fleet
Financing Receivable, Impaired [Line Items]
Impaired loans with a specific reserve		14.7		22.6
Associated allowance		(0.6)		(1.6)
Net carrying amount of impaired loans with specific reserve		14.1		21.0
Impaired loans with no specific reserve		6.0
Net carrying amount of impaired loans		20.1		21.0
Average amortized cost basis		23.7	[1]	24.5	[2]
Customer Retail | Owner/Operator
Financing Receivable, Impaired [Line Items]
Impaired loans with a specific reserve		1.5		2.1
Associated allowance		(0.2)		(0.3)
Net carrying amount of impaired loans with specific reserve		1.3		1.8
Impaired loans with no specific reserve		0.1
Net carrying amount of impaired loans		1.4		1.8
Average amortized cost basis		$ 1.9	[1]	$ 3.0	[2]

[1] Represents the average during the 12 months ended September 30, 2021.

[2] Represents the average during the 12 months ended September 30, 2020.