Allowance for Credit Losses - Allowance for Credit Losses and Finance Receivables by Portfolio Segment (Parenthetical) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Finance Receivables, Net [Member] \| Dealer Products [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for credit losses related to off-balance sheet commitments	$ 34	$ 37

Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121611286&loc=d3e11927-110247

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82919202-210446

Name:	us-gaap_OffBalanceSheetCreditLossLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant