Amortized Cost Basis of Finance Receivables by Credit Quality Indicator and Portfolio Class (Detail) - Financial Services - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	$ 2,033.1
Financed Receivables, By Origination Year, 2020	3,752.9
Financed Receivables, By Origination Year, 2019	2,843.0
Financed Receivables, By Origination Year, 2018	1,736.5
Financed Receivables, By Origination Year, 2017	840.4
Financed Receivables, By Origination Year, 2016	381.5
Financed Receivables, By Origination Year, Prior	208.8
Finance Receivables, Total	11,796.2	$ 12,055.8
Dealer
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,033.1
Financed Receivables, By Origination Year, 2020	509.4
Financed Receivables, By Origination Year, 2019	485.4
Financed Receivables, By Origination Year, 2018	272.5
Financed Receivables, By Origination Year, 2017	181.5
Financed Receivables, By Origination Year, 2016	101.6
Financed Receivables, By Origination Year, Prior	132.0
Finance Receivables, Total	3,715.5
Dealer | Wholesale
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,012.4
Finance Receivables, Total	2,012.4	2,907.4
Dealer | Wholesale | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,004.5
Finance Receivables, Total	2,004.5
Dealer | Wholesale | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	7.9
Finance Receivables, Total	7.9
Dealer | Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	20.7
Financed Receivables, By Origination Year, 2020	509.4
Financed Receivables, By Origination Year, 2019	485.4
Financed Receivables, By Origination Year, 2018	272.5
Financed Receivables, By Origination Year, 2017	181.5
Financed Receivables, By Origination Year, 2016	101.6
Financed Receivables, By Origination Year, Prior	132.0
Finance Receivables, Total	1,703.1	1,645.6
Dealer | Retail | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	20.7
Financed Receivables, By Origination Year, 2020	504.2
Financed Receivables, By Origination Year, 2019	474.9
Financed Receivables, By Origination Year, 2018	268.0
Financed Receivables, By Origination Year, 2017	180.1
Financed Receivables, By Origination Year, 2016	100.4
Financed Receivables, By Origination Year, Prior	132.0
Finance Receivables, Total	1,680.3
Dealer | Retail | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	5.2
Financed Receivables, By Origination Year, 2019	10.5
Financed Receivables, By Origination Year, 2018	4.5
Financed Receivables, By Origination Year, 2017	1.4
Finance Receivables, Total	21.6
Dealer | Retail | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2016	1.2
Finance Receivables, Total	1.2
Customer Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	3,243.5
Financed Receivables, By Origination Year, 2019	2,357.6
Financed Receivables, By Origination Year, 2018	1,464.0
Financed Receivables, By Origination Year, 2017	658.9
Financed Receivables, By Origination Year, 2016	279.9
Financed Receivables, By Origination Year, Prior	76.8
Finance Receivables, Total	8,080.7
Customer Retail | Fleet
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	2,686.4
Financed Receivables, By Origination Year, 2019	1,976.4
Financed Receivables, By Origination Year, 2018	1,233.7
Financed Receivables, By Origination Year, 2017	551.8
Financed Receivables, By Origination Year, 2016	238.1
Financed Receivables, By Origination Year, Prior	67.2
Finance Receivables, Total	6,753.6	6,347.7
Customer Retail | Fleet | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	2,664.9
Financed Receivables, By Origination Year, 2019	1,921.6
Financed Receivables, By Origination Year, 2018	1,203.7
Financed Receivables, By Origination Year, 2017	533.7
Financed Receivables, By Origination Year, 2016	234.2
Financed Receivables, By Origination Year, Prior	65.2
Finance Receivables, Total	6,623.3
Customer Retail | Fleet | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	13.5
Financed Receivables, By Origination Year, 2019	17.8
Financed Receivables, By Origination Year, 2018	11.8
Financed Receivables, By Origination Year, 2017	5.9
Financed Receivables, By Origination Year, 2016	1.5
Financed Receivables, By Origination Year, Prior	1.2
Finance Receivables, Total	51.7
Customer Retail | Fleet | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	8.0
Financed Receivables, By Origination Year, 2019	37.0
Financed Receivables, By Origination Year, 2018	18.2
Financed Receivables, By Origination Year, 2017	12.2
Financed Receivables, By Origination Year, 2016	2.4
Financed Receivables, By Origination Year, Prior	0.8
Finance Receivables, Total	78.6
Customer Retail | Owner/Operator
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	557.1
Financed Receivables, By Origination Year, 2019	381.2
Financed Receivables, By Origination Year, 2018	230.3
Financed Receivables, By Origination Year, 2017	107.1
Financed Receivables, By Origination Year, 2016	41.8
Financed Receivables, By Origination Year, Prior	9.6
Finance Receivables, Total	1,327.1	$ 1,155.1
Customer Retail | Owner/Operator | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	554.6
Financed Receivables, By Origination Year, 2019	376.4
Financed Receivables, By Origination Year, 2018	225.1
Financed Receivables, By Origination Year, 2017	105.2
Financed Receivables, By Origination Year, 2016	41.2
Financed Receivables, By Origination Year, Prior	9.1
Finance Receivables, Total	1,311.6
Customer Retail | Owner/Operator | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	1.6
Financed Receivables, By Origination Year, 2019	2.7
Financed Receivables, By Origination Year, 2018	2.0
Financed Receivables, By Origination Year, 2017	0.7
Financed Receivables, By Origination Year, 2016	0.2
Financed Receivables, By Origination Year, Prior	0.2
Finance Receivables, Total	7.4
Customer Retail | Owner/Operator | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	0.9
Financed Receivables, By Origination Year, 2019	2.1
Financed Receivables, By Origination Year, 2018	3.2
Financed Receivables, By Origination Year, 2017	1.2
Financed Receivables, By Origination Year, 2016	0.4
Financed Receivables, By Origination Year, Prior	0.3
Finance Receivables, Total	$ 8.1