Assets of nonconsolidated variable interest entities

No definition available.

Name:	ally_AssetsOfNonconsolidatedVariableInterestEntities
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the assets in the reporting entity's statement of financial position that relate to consolidated and nonconsolidated Variable Interest Entities.

+ References

No definition available.

+ Details

Name:	ally_CarryingvalueofassetsforVIEsforwhichwehavecontinuinginvolvement
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying value of liabilities for VIEs for which we have continuing involvement

+ References

No definition available.

+ Details

Name:	ally_CarryingvalueofliabilitiesforVIEsforwhichwehavecontinuinginvolvement
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107207-111719

+ Details

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

- Definition

Reflects, by transferred financial asset type, the amount of sales proceeds in excess of or deficient from the sum of the carrying amounts of transferred financial assets plus transaction costs, including those transactions in which the seller/transferor has continuing involvement with the financial assets that have been transferred.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=114780032&loc=d3e96813-111708

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=d3e107207-111719

+ Details

Name:	us-gaap_SecuritizationFinancialAssetForWhichTransferIsAccountedAsSaleGainLossOnSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685

+ Details

Name:	us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position.

+ References

+ Details

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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

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

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

Name:	us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount of the liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_HeldtomaturitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ally_ClassificationAxis=ally_UnencumberedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=ally_AmountReconsolidatedDuringthePeriodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=ally_AmountDeconsolidatedDuringthePeriodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Securitizations and Variable Interest Entities (Schedule of Variable Interest Entities) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Securitization or asset-backed financing arrangement, financial asset for which transfer is accounted as sale, gain on sale	$ 0	$ 1	$ 2
Variable interest entity, assets sold to nonconsolidated variable interest entities		(1,462)		$ (1,964)
Variable interest entity, nonconsolidated, maximum exposure to loss		(2,502)		(2,791)
Variable interest entity, carrying value of assets for which we have continuing involvement		27,757		30,776
Variable interest entity, carrying value of liabilities for which we have continuing involvement		11,853		10,483
Commercial portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, nonconsolidated, carrying amount, assets		762		592
Variable Interest Entity, nonconsolidated, carrying amount, liabilities		346		248
Variable interest entity, assets sold to nonconsolidated variable interest entities		0		0
Variable interest entity, nonconsolidated, maximum exposure to loss		(988)		(790)
Automobile loan \| Consumer portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets		16,982		17,597
Variable interest entity, consolidated, carrying amount, liabilities		7,113		7,677
Variable interest entity, nonconsolidated, carrying amount, assets		52		37
Variable Interest Entity, nonconsolidated, carrying amount, liabilities		0		0
Variable interest entity, assets sold to nonconsolidated variable interest entities		(1,462)		(1,964)
Variable interest entity, nonconsolidated, maximum exposure to loss		(1,514)		(2,001)
Cash flows between transferor and transferee, proceeds from transfers completed during the period		24	$ 1,187
Automobile loan \| Consumer portfolio segment \| Held-to-maturity securities
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets		49		36
Automobile loan \| Consumer portfolio segment \| Other assets
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets		3		1
Automobile loan \| Commercial portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets		9,961		12,550
Variable interest entity, consolidated, carrying amount, liabilities		4,394		2,558
Unencumbered \| Automobile loan \| Consumer portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets		8,500		8,500
Variable interest entity, consolidated, carrying amount, liabilities		24		$ 29
Amount Reconsolidated During the Period [Member] \| Automobile loan \| Consumer portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, assets sold to nonconsolidated variable interest entities		223
Variable interest entity, nonconsolidated, maximum exposure to loss		219
Amount Deconsolidated During the Period [Member] \| Automobile loan \| Consumer portfolio segment
Variable Interest Entity [Line Items]
Variable interest entity, assets sold to nonconsolidated variable interest entities		(545)
Variable interest entity, nonconsolidated, maximum exposure to loss		$ (497)