Investment Securities (Schedule of Unrealized Loss on Investments) (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Less than 12 months
Fair value	$ 590,000,000	$ 12,548,000,000
Unrealized loss	(10,000,000)	(1,503,000,000)
12 months or longer
Fair value	23,331,000,000	16,660,000,000
Unrealized loss	(4,000,000,000)	(3,824,000,000)
Credit reserves	0	0
U.S. Treasury and federal agencies
Less than 12 months
Fair value	0	529,000,000
Unrealized loss	0	(68,000,000)
12 months or longer
Fair value	2,075,000,000	1,487,000,000
Unrealized loss	(209,000,000)	(188,000,000)
U.S. States and political subdivisions
Less than 12 months
Fair value	70,000,000	547,000,000
Unrealized loss	0	(55,000,000)
12 months or longer
Fair value	501,000,000	135,000,000
Unrealized loss	(70,000,000)	(27,000,000)
Foreign government
Less than 12 months
Fair value	16,000,000	75,000,000
Unrealized loss	0	(4,000,000)
12 months or longer
Fair value	134,000,000	71,000,000
Unrealized loss	(8,000,000)	(8,000,000)
Agency mortgage-backed residential
Less than 12 months
Fair value	300,000,000	7,472,000,000
Unrealized loss	(5,000,000)	(892,000,000)
12 months or longer
Fair value	15,015,000,000	8,978,000,000
Unrealized loss	(2,734,000,000)	(2,146,000,000)
Mortgage-backed residential
Less than 12 months
Fair value	0	1,985,000,000
Unrealized loss	0	(289,000,000)
12 months or longer
Fair value	225,000,000	2,287,000,000
Unrealized loss	(43,000,000)	(566,000,000)
Agency mortgage-backed commercial
Less than 12 months
Fair value	153,000,000	996,000,000
Unrealized loss	(4,000,000)	(124,000,000)
12 months or longer
Fair value	3,472,000,000	2,535,000,000
Unrealized loss	(779,000,000)	(721,000,000)
Asset-backed
Less than 12 months
Fair value	18,000,000	162,000,000
Unrealized loss	0	(4,000,000)
12 months or longer
Fair value	302,000,000	272,000,000
Unrealized loss	(12,000,000)	(22,000,000)
Corporate debt
Less than 12 months
Fair value	33,000,000	782,000,000
Unrealized loss	(1,000,000)	(67,000,000)
12 months or longer
Fair value	1,607,000,000	895,000,000
Unrealized loss	$ (145,000,000)	$ (146,000,000)

Debt Securities, Available-for-Sale, Unrealized Loss Position, Long

No definition available.

Name:	ally_DebtSecuritiesAvailableForSaleUnrealizedLossPositionLongAbstract
Namespace Prefix:	ally_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ally_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: