Leasing (Ally as the Lessor) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Lessor, Lease, Description [Line Items]
Residual value guarantee, percentage	15.00%	15.00%
Vehicles	$ 12,445	$ 11,182
Accumulated depreciation	(1,476)	(1,543)
Investment in operating leases, net	$ 10,969	9,639
Minimum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, lessee option to purchase underlying asset	24
Maximum
Lessor, Lease, Description [Line Items]
Lessor, operating lease, lessee option to purchase underlying asset	60 months
Vehicles
Lessor, Lease, Description [Line Items]
Residual value of leased asset	$ 211	$ 352

Amount of accumulated depreciation on property leased by the lessor under an operating lease.

No definition available.

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

- Definition

Gross amount of property leased by the lessor under an operating lease.

+ References

No definition available.

+ Details

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

- Definition

Amount of net investment in operating leases of lessor.

+ References

No definition available.

+ Details

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

- Definition

Residual value guarantee

+ References

No definition available.

+ Details

Name:	ally_Residualvalueguarantee
Namespace Prefix:	ally_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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_LessorLeaseDescriptionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of terms and conditions for lessee to purchase underlying asset related to operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=124258985&loc=SL77919359-209981

+ Details

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

- Definition

Amount of residual asset covered by residual value guarantee under operating lease. Excludes guarantee considered to be lease payments for lessor.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=124258985&loc=SL77919359-209981

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
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Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: