Investment Securities (Investment Table) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt securities, available-for-sale, fair value	$ 29,384	$ 25,303
Debt securities, held-to-maturity, amortized cost	2,618	2,362
Held-to-maturity securities, fair value	2,687	2,307
Available-for-sale securities
Schedule of Investments [Line Items]
Securities pledged for Federal Home Loan Bank, at fair value	2,500	9,200
Securities, amount eligible to be repledged or sold by counterparty	594	821
Operating Segments \| Insurance operations \| Available-for-sale securities
Schedule of Investments [Line Items]
Deposit securities	12	12
US Treasury \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	2,389	1,911
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(15)	(60)
Debt securities, available-for-sale, fair value	2,382	1,851
US States and political subdivisions \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	612	816
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	19	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1)	(17)
Debt securities, available-for-sale, fair value	630	802
Foreign government \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	152	145
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	(1)
Debt securities, available-for-sale, fair value	155	145
Agency mortgage-backed residential \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	19,887	17,486
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	244	47
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(23)	(395)
Debt securities, available-for-sale, fair value	20,108	17,138
Agency mortgage-backed residential \| Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost	2,593	2,319
Debt securities, held-to-maturity, unrecognized gain	70	6
Debt securities, held-to-maturity, unrecognized loss	1	61
Held-to-maturity securities, fair value	2,662	2,264
Mortgage-backed residential \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	2,799	2,796
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	22	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(9)	(111)
Debt securities, available-for-sale, fair value	2,812	2,686
Agency mortgage-backed commercial \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	1,341	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	72	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Debt securities, available-for-sale, fair value	1,413	3
Mortgage-backed commercial \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	112	715
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	(2)
Debt securities, available-for-sale, fair value	113	714
Asset-backed \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	413	723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	(2)
Debt securities, available-for-sale, fair value	417	723
Asset-backed \| Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost	25	43
Debt securities, held-to-maturity, unrecognized gain	0	0
Debt securities, held-to-maturity, unrecognized loss	0	0
Held-to-maturity securities, fair value	25	43
Corporate debt \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	1,321	1,286
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	35	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(2)	(46)
Debt securities, available-for-sale, fair value	1,354	1,241
Available-for-sale debt securities \| Available-for-sale securities
Schedule of Investments [Line Items]
Debt securities, available-for-sale, amortized cost	29,026	25,881
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	408	56
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(50)	(634)
Debt securities, available-for-sale, fair value	29,384	25,303
Held-to-maturity securities
Schedule of Investments [Line Items]
Debt securities, held-to-maturity, amortized cost	2,618	2,362
Debt securities, held-to-maturity, unrecognized gain	70	6
Debt securities, held-to-maturity, unrecognized loss	1	61
Held-to-maturity securities, fair value	2,687	2,307
Federal Home Loan Bank certificates and obligations \| Held-to-maturity securities
Schedule of Investments [Line Items]
Securities pledged for Federal Home Loan Bank, at fair value	$ 972	$ 1,200

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions where counterparty has the right to re-pledge such.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

+ Details

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

- Definition

The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of the FHLB debt obligations, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

+ Details

Name:	us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForFederalHomeLoanBankAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfInvestmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ally_InsuranceOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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_InvestmentTypeAxis=us-gaap_HeldtomaturitySecuritiesMember
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_AgencymortgagebackedcommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
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:

- Details

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

- Details

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

- Details

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