Amortized Cost Basis of Finance Receivables by Credit Quality Indicator and Portfolio Class (Detail) - Financial Services $ in Millions	Mar. 31, 2020 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	$ 811.5
Financed Receivables, By Origination Year, 2019	3,424.4
Financed Receivables, By Origination Year, 2018	2,276.6
Financed Receivables, By Origination Year, 2017	1,198.4
Financed Receivables, By Origination Year, 2016	639.7
Financed Receivables, By Origination Year, Prior	452.5
Financed Receivables, By Origination Year, Loans	2,888.9
Finance Receivables, Total	11,692.0
Dealer
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	111.8
Financed Receivables, By Origination Year, 2019	562.0
Financed Receivables, By Origination Year, 2018	374.7
Financed Receivables, By Origination Year, 2017	211.9
Financed Receivables, By Origination Year, 2016	128.3
Financed Receivables, By Origination Year, Prior	228.4
Financed Receivables, By Origination Year, Loans	2,888.9
Finance Receivables, Total	4,506.0
Dealer | Wholesale
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,875.7
Finance Receivables, Total	2,875.7
Dealer | Wholesale | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,803.4
Finance Receivables, Total	2,803.4
Dealer | Wholesale | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	72.3
Finance Receivables, Total	72.3
Dealer | Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	111.8
Financed Receivables, By Origination Year, 2019	562.0
Financed Receivables, By Origination Year, 2018	374.7
Financed Receivables, By Origination Year, 2017	211.9
Financed Receivables, By Origination Year, 2016	128.3
Financed Receivables, By Origination Year, Prior	228.4
Financed Receivables, By Origination Year, Loans	13.2
Finance Receivables, Total	1,630.3
Dealer | Retail | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	110.2
Financed Receivables, By Origination Year, 2019	550.4
Financed Receivables, By Origination Year, 2018	366.8
Financed Receivables, By Origination Year, 2017	210.5
Financed Receivables, By Origination Year, 2016	128.3
Financed Receivables, By Origination Year, Prior	228.4
Financed Receivables, By Origination Year, Loans	13.2
Finance Receivables, Total	1,607.8
Dealer | Retail | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	1.6
Financed Receivables, By Origination Year, 2019	11.6
Financed Receivables, By Origination Year, 2018	6.2
Financed Receivables, By Origination Year, 2017	1.4
Finance Receivables, Total	20.8
Dealer | Retail | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2018	1.7
Finance Receivables, Total	1.7
Customer Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	699.7
Financed Receivables, By Origination Year, 2019	2,862.4
Financed Receivables, By Origination Year, 2018	1,901.9
Financed Receivables, By Origination Year, 2017	986.5
Financed Receivables, By Origination Year, 2016	511.4
Financed Receivables, By Origination Year, Prior	224.1
Finance Receivables, Total	7,186.0
Customer Retail | Fleet
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	608.8
Financed Receivables, By Origination Year, 2019	2,429.5
Financed Receivables, By Origination Year, 2018	1,608.1
Financed Receivables, By Origination Year, 2017	838.4
Financed Receivables, By Origination Year, 2016	440.3
Financed Receivables, By Origination Year, Prior	197.5
Finance Receivables, Total	6,122.6
Customer Retail | Fleet | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	603.2
Financed Receivables, By Origination Year, 2019	2,388.9
Financed Receivables, By Origination Year, 2018	1,559.2
Financed Receivables, By Origination Year, 2017	809.9
Financed Receivables, By Origination Year, 2016	428.6
Financed Receivables, By Origination Year, Prior	192.6
Finance Receivables, Total	5,982.4
Customer Retail | Fleet | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	3.7
Financed Receivables, By Origination Year, 2019	23.2
Financed Receivables, By Origination Year, 2018	29.0
Financed Receivables, By Origination Year, 2017	19.5
Financed Receivables, By Origination Year, 2016	6.6
Financed Receivables, By Origination Year, Prior	2.9
Finance Receivables, Total	84.9
Customer Retail | Fleet | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	1.9
Financed Receivables, By Origination Year, 2019	17.4
Financed Receivables, By Origination Year, 2018	19.9
Financed Receivables, By Origination Year, 2017	9.0
Financed Receivables, By Origination Year, 2016	5.1
Financed Receivables, By Origination Year, Prior	2.0
Finance Receivables, Total	55.3
Customer Retail | Owner/Operator
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	90.9
Financed Receivables, By Origination Year, 2019	432.9
Financed Receivables, By Origination Year, 2018	293.8
Financed Receivables, By Origination Year, 2017	148.1
Financed Receivables, By Origination Year, 2016	71.1
Financed Receivables, By Origination Year, Prior	26.6
Finance Receivables, Total	1,063.4
Customer Retail | Owner/Operator | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	90.5
Financed Receivables, By Origination Year, 2019	426.9
Financed Receivables, By Origination Year, 2018	287.5
Financed Receivables, By Origination Year, 2017	145.0
Financed Receivables, By Origination Year, 2016	69.5
Financed Receivables, By Origination Year, Prior	25.3
Finance Receivables, Total	1,044.7
Customer Retail | Owner/Operator | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	0.4
Financed Receivables, By Origination Year, 2019	4.7
Financed Receivables, By Origination Year, 2018	4.1
Financed Receivables, By Origination Year, 2017	2.3
Financed Receivables, By Origination Year, 2016	0.8
Financed Receivables, By Origination Year, Prior	0.4
Finance Receivables, Total	12.7
Customer Retail | Owner/Operator | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2019	1.3
Financed Receivables, By Origination Year, 2018	2.2
Financed Receivables, By Origination Year, 2017	0.8
Financed Receivables, By Origination Year, 2016	0.8
Financed Receivables, By Origination Year, Prior	0.9
Finance Receivables, Total	$ 6.0