Finance Receivables and Loans, Net (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	$ 1,242	$ 1,276
Allowance for loan and lease losses, charge-offs	(1,490)	(1,433)
Allowance for loan and lease losses, recoveries	514	488
Allowance for loan and lease losses, net charge-offs	(976)	(945)
Provision for loan losses	998	918	$ 1,148
Allowance for Loan and Lease Losses, Adjustments, Other	(1)	(7)
Allowance for loan losses	1,263	1,242	1,276
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	89	123
Allowance for loan losses, collectively evaluated for impairment	1,174	1,119
Finance receivables and loans, net	128,231	129,926
Finance receivables and loans, individually evaluated for impairment	961	1,075
Finance receivables and loans, collectively evaluated for impairment		128,851
Excludes fair value option elected other loans
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	128,220
Finance receivables and loans, collectively evaluated for impairment	127,259
Consumer portfolio segment
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	89,924	87,240
Consumer portfolio segment \| Excludes fair value option elected other loans
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	89,913
Consumer portfolio segment \| Fair value \| Finance receivables and Loans, net
Financing Receivable, Allowance for Credit Loss [Line Items]
Loans Receivable, Fair Value Disclosure	11
Consumer portfolio segment \| Automotive loan
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	1,048	1,066
Allowance for loan and lease losses, charge-offs	(1,423)	(1,383)
Allowance for loan and lease losses, recoveries	493	456
Allowance for loan and lease losses, net charge-offs	(930)	(927)
Provision for loan losses	957	911
Allowance for Loan and Lease Losses, Adjustments, Other	0	(2)
Allowance for loan losses	1,075	1,048	1,066
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	38	44
Allowance for loan losses, collectively evaluated for impairment	1,037	1,004
Finance receivables and loans, net	72,390	70,539
Finance receivables and loans, individually evaluated for impairment	538	495
Finance receivables and loans, collectively evaluated for impairment	71,852	70,044
Consumer portfolio segment \| Real estate-backed loan
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	53	79
Allowance for loan and lease losses, charge-offs	(13)	(35)
Allowance for loan and lease losses, recoveries	21	25
Allowance for loan and lease losses, net charge-offs	8	(10)
Provision for loan losses	(13)	(15)
Allowance for Loan and Lease Losses, Adjustments, Other	(2)	(1)
Allowance for loan losses	46	53	79
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	18	23
Allowance for loan losses, collectively evaluated for impairment	28	30
Finance receivables and loans, net	17,322	16,701
Finance receivables and loans, individually evaluated for impairment	208	231
Finance receivables and loans, collectively evaluated for impairment	17,114	16,470
Consumer portfolio segment \| Other
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	0
Allowance for loan and lease losses, charge-offs	(5)
Allowance for loan and lease losses, recoveries	0
Allowance for loan and lease losses, net charge-offs	(5)
Provision for loan losses	14
Allowance for Loan and Lease Losses, Adjustments, Other	0
Allowance for loan losses	9	0
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	0
Allowance for loan losses, collectively evaluated for impairment	9
Finance receivables and loans, individually evaluated for impairment	0
Consumer portfolio segment \| Other \| Excludes fair value option elected other loans
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	201
Finance receivables and loans, collectively evaluated for impairment	201
Commercial portfolio segment
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	141	131
Allowance for loan and lease losses, charge-offs	(49)	(15)
Allowance for loan and lease losses, recoveries	0	7
Allowance for loan and lease losses, net charge-offs	(49)	(8)
Provision for loan losses	40	22
Allowance for Loan and Lease Losses, Adjustments, Other	1	(4)
Allowance for loan losses	133	141	$ 131
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	33	56
Allowance for loan losses, collectively evaluated for impairment	100	85
Finance receivables and loans, net	38,307	42,686
Finance receivables and loans, individually evaluated for impairment	215	349
Finance receivables and loans, collectively evaluated for impairment	38,092	42,337
Commercial portfolio segment \| Automotive loan
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	28,332	33,672
Commercial portfolio segment \| Real estate-backed loan
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Finance receivables and loans, net	$ 4,961	$ 4,809

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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

- Definition

Amount of other addition (reduction) in allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount, after recoveries, of loans and leases that have been written-off.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
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_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=ally_ExcludesfairvalueoptionelectedotherloansMember
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_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ally_FinancereceivablesandLoansnetMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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