Securitizations and Variable Interest Entities (Schedule of Cash Flows with Nonconsolidated Special-Purpose Entities) (Details) - Off-balance sheet variable interest entities - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021
Cash Flow Received and Paid to Nonconsolidated Securitization Entities [Line Items]
Total	$ 51	$ 0
Consumer Automotive Industry Sector
Cash Flow Received and Paid to Nonconsolidated Securitization Entities [Line Items]
Cash proceeds from transfers completed during the period	2	0
Consumer Other Portfolio Sector
Cash Flow Received and Paid to Nonconsolidated Securitization Entities [Line Items]
Cash proceeds from transfers completed during the period	45	0
Servicing fees	$ 4	$ 0

Cash Flows between a transferor and a transferee attributable to newly transferred assets related to either a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(1)
-URI https://asc.fasb.org/extlink&oid=121570589&loc=d3e107207-111719

+ Details

Name:	us-gaap_CashFlowsBetweenTransfereeAndTransferorProceedsFromNewTransfers
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Cash Flows between a transferee and a transferor attributable to servicing fees related to a securitization, asset-backed financing arrangement, or similar transfer in which the transferor has continuing involvement with the transferred financial assets underlying the transaction (including, but not limited to, servicing, recourse, and restrictions on transferor's interests in the transferred financial assets).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(4)
-URI https://asc.fasb.org/extlink&oid=121570589&loc=d3e107207-111719

+ Details

Name:	us-gaap_CashFlowsBetweenTransfereeAndTransferorServicingFees
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow from collection on beneficial interest in securitization of receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromCollectionOfRetainedInterestInSecuritizedReceivables
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_TransferOfFinancialAssetsAccountedForAsSalesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=ally_ConsumerAutomotiveIndustrySectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EquitySecuritiesByIndustryAxis=ally_ConsumerOtherPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: