Finance Receivables And Loans, Net (Allowance For Credit Losses On Financing Receivables) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	$ 977	$ 1,208
Allowance for Loan and Lease Losses, Write-offs	(892)	(776)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	283	239
Allowance for Loan and Lease Losses Write-offs, Net	(609)	(537)
Provision for Loan and Lease Losses	707	457
Allowance for Loan and Lease Losses, Adjustments, Net	(21)	(151)
Loans and Leases Receivable, Allowance, Ending Balance	1,054	977
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, Individually evaluated for impairment	86	106
Allowance for loan losses, Collectively evaluated for impairment	968	871
Finance receivables and loans, net	111,600	99,948
Loans and Leases Receivable, Gross, Excluding Fair Value Option Election	111,600	99,947
Financing Receivable, Individually Evaluated for Impairment	658	700
Financing receivable, Collectively evaluated for impairment	110,942	99,246
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses	0	0
Finance receivables and loans, net	0	1
Consumer Portfolio Segment [Member] \| Automobile Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	685	673
Allowance for Loan and Lease Losses, Write-offs	(840)	(720)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	262	219
Allowance for Loan and Lease Losses Write-offs, Net	(578)	(501)
Provision for Loan and Lease Losses	739	540
Allowance for Loan and Lease Losses, Adjustments, Net	(12)	(27)
Loans and Leases Receivable, Allowance, Ending Balance	834	685
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, Individually evaluated for impairment	22	23
Allowance for loan losses, Collectively evaluated for impairment	812	662
Finance receivables and loans, net	64,292	56,570
Loans and Leases Receivable, Gross, Excluding Fair Value Option Election		56,570
Financing Receivable, Individually Evaluated for Impairment	315	282
Financing receivable, Collectively evaluated for impairment	63,977	56,287
Consumer Portfolio Segment [Member] \| Automobile Loan [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses	0	0
Finance receivables and loans, net	0	1
Consumer Portfolio Segment [Member] \| Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	152	389
Allowance for Loan and Lease Losses, Write-offs	(48)	(51)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	17	8
Allowance for Loan and Lease Losses Write-offs, Net	(31)	(43)
Provision for Loan and Lease Losses	1	(69)
Allowance for Loan and Lease Losses, Adjustments, Net	(8)	(125)
Loans and Leases Receivable, Allowance, Ending Balance	114	152
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, Individually evaluated for impairment	44	62
Allowance for loan losses, Collectively evaluated for impairment	70	90
Finance receivables and loans, net	9,773	7,474
Loans and Leases Receivable, Gross, Excluding Fair Value Option Election		7,473
Financing Receivable, Individually Evaluated for Impairment	266	336
Financing receivable, Collectively evaluated for impairment	9,507	7,137
Consumer Portfolio Segment [Member] \| Residential Mortgage [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses	0	0
Finance receivables and loans, net	0	0
Commercial Portfolio Segment [Member] \| Commercial Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans and Leases Receivable, Allowance, Beginning Balance	140	146
Allowance for Loan and Lease Losses, Write-offs	(4)	(5)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	4	12
Allowance for Loan and Lease Losses Write-offs, Net	0	7
Provision for Loan and Lease Losses	(33)	(14)
Allowance for Loan and Lease Losses, Adjustments, Net	(1)	1
Loans and Leases Receivable, Allowance, Ending Balance	106	140
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, Individually evaluated for impairment	20	21
Allowance for loan losses, Collectively evaluated for impairment	86	119
Finance receivables and loans, net	37,535	35,904
Loans and Leases Receivable, Gross, Excluding Fair Value Option Election		35,904
Financing Receivable, Individually Evaluated for Impairment	77	82
Financing receivable, Collectively evaluated for impairment	37,458	35,822
Commercial Portfolio Segment [Member] \| Commercial Loan [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses	0	0
Finance receivables and loans, net	$ 0	$ 0

Loans and Leases Receivable, Gross, Excluding Fair Value Option Election

No definition available.

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

- Definition

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

+ References

+ Details

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

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

+ References

+ 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

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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 4: http://www.xbrl.org/2003/role/presentationRef
-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=6379141&loc=d3e15032-111544

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 6
-Section L
-Subsection 1

+ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524

+ Details

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

- Details

Name:	us-gaap_ReceivablesAcquiredWithDeterioratedCreditQualityAxis=us-gaap_ReceivablesAcquiredWithDeterioratedCreditQualityMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
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:

- Details

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