Derivative Instruments and Hedging Activities (Fair Value Amounts of Derivative Instruments Reported on our Condensed Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	$ 11	$ 7
Fair value of derivative contracts in payable position	48	62
Derivative, notional amount	29,186	18,169
Credit derivative, maximum exposure, undiscounted	94	119
Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	8	0
Fair value of derivative contracts in payable position	0	2
Derivative, notional amount	28,810	17,210
Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	3	7
Fair value of derivative contracts in payable position	48	60
Derivative, notional amount	376	959
Interest rate contracts \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	1	6
Fair value of derivative contracts in payable position	2	2
Derivative, notional amount	235	803
Interest rate swaps \| Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	0
Fair value of derivative contracts in payable position	0	0
Derivative, notional amount	28,218	17,039
Interest rate futures and forwards \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	1	1
Fair value of derivative contracts in payable position	0	0
Derivative, notional amount	108	223
Interest rate written options \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	5
Fair value of derivative contracts in payable position	2	2
Derivative, notional amount	127	580
Foreign exchange contracts \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	2	0
Fair value of derivative contracts in payable position	0	1
Derivative, notional amount	141	154
Foreign exchange forwards \| Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	5	0
Fair value of derivative contracts in payable position	0	2
Derivative, notional amount	142	171
Foreign exchange futures and forwards \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	2	0
Fair value of derivative contracts in payable position	0	1
Derivative, notional amount	141	154
Other credit derivatives \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	0
Fair value of derivative contracts in payable position	46	56
Equity contracts \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	1
Fair value of derivative contracts in payable position	0	1
Derivative, notional amount	0	2
Equity contract written options \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	0
Fair value of derivative contracts in payable position	0	1
Derivative, notional amount	0	2
Purchased Options \| Designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	3	0
Fair value of derivative contracts in payable position	0	0
Derivative, notional amount	450	0
Purchased Options \| Not designated as hedging instrument
Derivatives, Fair Value [Line Items]
Fair value of derivative contracts in receivable position	0	1
Fair value of derivative contracts in payable position	0	0
Derivative, notional amount	$ 0	$ 0

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (b)(1)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5708775-113959

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

+ References

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
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_DerivativesFairValueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ally_InterestRateFuturesAndForwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ally_InterestRatePutOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ally_ForeignExchangeFuturesAndForwardsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherCreditDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EquityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ally_EquityContractPutOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ally_PurchasedOptionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: