Amortized Cost Basis of Finance Receivables by Credit Quality Indicator and Portfolio Class (Detail) - Financial Services - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	$ 2,103.3
Financed Receivables, By Origination Year, 2020	2,557.1
Financed Receivables, By Origination Year, 2019	3,024.9
Financed Receivables, By Origination Year, 2018	1,889.1
Financed Receivables, By Origination Year, 2017	980.2
Financed Receivables, By Origination Year, 2016	456.7
Financed Receivables, By Origination Year, Prior	282.0
Finance Receivables, Total	11,293.3	$ 12,055.8
Dealer
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,103.3
Financed Receivables, By Origination Year, 2020	347.9
Financed Receivables, By Origination Year, 2019	519.8
Financed Receivables, By Origination Year, 2018	290.7
Financed Receivables, By Origination Year, 2017	193.9
Financed Receivables, By Origination Year, 2016	111.6
Financed Receivables, By Origination Year, Prior	162.7
Finance Receivables, Total	3,729.9
Dealer | Wholesale
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,079.7
Finance Receivables, Total	2,079.7	2,907.4
Dealer | Wholesale | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	2,066.3
Finance Receivables, Total	2,066.3
Dealer | Wholesale | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	13.4
Finance Receivables, Total	13.4
Dealer | Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	23.6
Financed Receivables, By Origination Year, 2020	347.9
Financed Receivables, By Origination Year, 2019	519.8
Financed Receivables, By Origination Year, 2018	290.7
Financed Receivables, By Origination Year, 2017	193.9
Financed Receivables, By Origination Year, 2016	111.6
Financed Receivables, By Origination Year, Prior	162.7
Finance Receivables, Total	1,650.2	1,645.6
Dealer | Retail | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, Loans	23.6
Financed Receivables, By Origination Year, 2020	345.1
Financed Receivables, By Origination Year, 2019	508.8
Financed Receivables, By Origination Year, 2018	285.0
Financed Receivables, By Origination Year, 2017	192.4
Financed Receivables, By Origination Year, 2016	110.1
Financed Receivables, By Origination Year, Prior	162.7
Finance Receivables, Total	1,627.7
Dealer | Retail | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	2.8
Financed Receivables, By Origination Year, 2019	11.0
Financed Receivables, By Origination Year, 2018	5.7
Financed Receivables, By Origination Year, 2017	1.5
Finance Receivables, Total	21.0
Dealer | Retail | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2016	1.5
Finance Receivables, Total	1.5
Customer Retail
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	2,209.2
Financed Receivables, By Origination Year, 2019	2,505.1
Financed Receivables, By Origination Year, 2018	1,598.4
Financed Receivables, By Origination Year, 2017	786.3
Financed Receivables, By Origination Year, 2016	345.1
Financed Receivables, By Origination Year, Prior	119.3
Finance Receivables, Total	7,563.4
Customer Retail | Fleet
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	1,816.7
Financed Receivables, By Origination Year, 2019	2,109.5
Financed Receivables, By Origination Year, 2018	1,343.3
Financed Receivables, By Origination Year, 2017	658.8
Financed Receivables, By Origination Year, 2016	288.5
Financed Receivables, By Origination Year, Prior	103.0
Finance Receivables, Total	6,319.8	6,347.7
Customer Retail | Fleet | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	1,787.1
Financed Receivables, By Origination Year, 2019	2,058.4
Financed Receivables, By Origination Year, 2018	1,306.4
Financed Receivables, By Origination Year, 2017	632.2
Financed Receivables, By Origination Year, 2016	283.7
Financed Receivables, By Origination Year, Prior	100.1
Finance Receivables, Total	6,167.9
Customer Retail | Fleet | Watch
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	12.2
Financed Receivables, By Origination Year, 2019	31.2
Financed Receivables, By Origination Year, 2018	22.5
Financed Receivables, By Origination Year, 2017	18.4
Financed Receivables, By Origination Year, 2016	3.0
Financed Receivables, By Origination Year, Prior	1.8
Finance Receivables, Total	89.1
Customer Retail | Fleet | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	17.4
Financed Receivables, By Origination Year, 2019	19.9
Financed Receivables, By Origination Year, 2018	14.4
Financed Receivables, By Origination Year, 2017	8.2
Financed Receivables, By Origination Year, 2016	1.8
Financed Receivables, By Origination Year, Prior	1.1
Finance Receivables, Total	62.8
Customer Retail | Owner/Operator
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	392.5
Financed Receivables, By Origination Year, 2019	395.6
Financed Receivables, By Origination Year, 2018	255.1
Financed Receivables, By Origination Year, 2017	127.5
Financed Receivables, By Origination Year, 2016	56.6
Financed Receivables, By Origination Year, Prior	16.3
Finance Receivables, Total	1,243.6	$ 1,155.1
Customer Retail | Owner/Operator | Performing
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	390.2
Financed Receivables, By Origination Year, 2019	389.0
Financed Receivables, By Origination Year, 2018	248.9
Financed Receivables, By Origination Year, 2017	124.4
Financed Receivables, By Origination Year, 2016	55.5
Financed Receivables, By Origination Year, Prior	15.7
Finance Receivables, Total	1,223.7
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	3.8
Financed Receivables, By Origination Year, 2018	2.4
Financed Receivables, By Origination Year, 2017	1.4
Financed Receivables, By Origination Year, 2016	0.5
Financed Receivables, By Origination Year, Prior	0.1
Finance Receivables, Total	9.8
Customer Retail | Owner/Operator | At-risk
Financing Receivable, Recorded Investment [Line Items]
Financed Receivables, By Origination Year, 2020	0.7
Financed Receivables, By Origination Year, 2019	2.8
Financed Receivables, By Origination Year, 2018	3.8
Financed Receivables, By Origination Year, 2017	1.7
Financed Receivables, By Origination Year, 2016	0.6
Financed Receivables, By Origination Year, Prior	0.5
Finance Receivables, Total	$ 10.1