Debt Debt (Schedule of Short-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Demand Notes	$ 3,171	$ 3,622
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	7,350	7,875
Securities Sold under Agreements to Repurchase, Gross	892	1,176
Short-term Debt	$ 11,413	$ 12,673
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.50%	1.00%
Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements	$ 10	$ 45
Non-derivative Cash Collateral received from counterparties associated with Repurchase Agreements	1
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,171	3,622
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	0	0
Securities Sold under Agreements to Repurchase, Gross	0	0
Short-term Debt	3,171	3,622
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Federal Home Loan Bank, Advances, Maturities Summary, Due in Next Twelve Months	7,350	7,875
Securities Sold under Agreements to Repurchase, Gross	892	1,176
Short-term Debt	8,242	9,051
Maturity less than 90 Days [Member] \| Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	892
Maturity Less than 30 Days [Member] \| Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	73
Maturity 31 to 60 Days [Member] \| Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	521
Maturity 61 to 90 Days [Member] \| Secured Debt [Member]
Short-term Debt [Line Items]
Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period	$ 298
One year, or less [Member] \| Secured Debt [Member]
Short-term Debt [Line Items]
Asset-backed Automotive Financial Securities Sold under Agreements to Repurchase, Maturity Period		$ 500

Asset-backed Automotive Financial Securities Sold under Agreements to Repurchase, Maturity Period

No definition available.

Name:	ally_AssetbackedAutomotiveFinancialSecuritiesSoldunderAgreementstoRepurchaseMaturityPeriod
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Demand Notes

+ References

No definition available.

+ Details

Name:	ally_DemandNotes
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Mortgage-backed Residential Securities Sold under Agreements to Repurchase, Maturity Period

+ References

No definition available.

+ Details

Name:	ally_MortgagebackedResidentialSecuritiesSoldunderAgreementstoRepurchaseMaturityPeriod
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Non-derivative Cash Collateral Placed with Counterparties Associated with the Repurchase Agreements

+ References

No definition available.

+ Details

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

- Definition

Non-derivative Cash Collateral received from counterparties associated with Repurchase Agreements

+ References

No definition available.

+ Details

Name:	ally_NonderivativeCashCollateralreceivedfromcounterpartiesassociatedwithRepurchaseAgreements
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

+ Details

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

- Definition

Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

Name:	us-gaap_ShortTermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=ally_Maturitylessthan90daysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityUpTo30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=ally_Maturity31to60DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=ally_Maturity61to90DaysMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=ally_OneyearorlessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: