Finance Receivables and Loans, Net (Schedule of Sales of Financing Receivables and Loans) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Sale	$ 128	$ 940	$ 128	$ 960
Financing receivable, sales and transfers				131
Loans held-for-sale transferred to finance receivables and loans held-for-investment				79
Consumer portfolio segment \| Automotive loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Sale	0	0	0	20
Consumer portfolio segment \| Real estate-backed loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Sale	$ 128	$ 940	$ 128	$ 940

Financing Receivable, sale initially classified as held-for-investment

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Loans held-for-sale transferred to finance receivables and loans held-for-investment

+ References

No definition available.

+ Details

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Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- 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

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- Definition

Amount of decrease from sale and reclassification to held-for-sale of 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 11
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919244-210447

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 (f)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

+ Details

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Period Type:	duration

- Details

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- Details

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- Details

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