Finance Receivables And Loans, Net Finance Receivables and Loans, Net (Troubled Debt Restructurings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	17,913	20,497
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 362	$ 712
Financing Receivable, Modifications, Post-Modification Recorded Investment	332	618
Financing Receivable, Modifications, Recorded Investment	681
Total TDR movement during the period	606ally_TotalTdrMovementDuringPeriod
Loans and Leases Receivable, Impaired, Commitment to Lend	4	26
Consumer Loans Auto Financing Receivable [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	17,511	19,388
Financing Receivable, Modifications, Pre-Modification Recorded Investment	211	297
Financing Receivable, Modifications, Post-Modification Recorded Investment	187	249
Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	396	1,092
Financing Receivable, Modifications, Pre-Modification Recorded Investment	80	278
Financing Receivable, Modifications, Post-Modification Recorded Investment	74	234
Commercial Loan [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	6	17
Financing Receivable, Modifications, Pre-Modification Recorded Investment	71	137
Financing Receivable, Modifications, Post-Modification Recorded Investment	71	135
Commercial And Industrial Automobile [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	3	8
Financing Receivable, Modifications, Pre-Modification Recorded Investment	23	37
Financing Receivable, Modifications, Post-Modification Recorded Investment	23	37
Commercial And Industrial Other [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	3	4
Financing Receivable, Modifications, Pre-Modification Recorded Investment	48	80
Financing Receivable, Modifications, Post-Modification Recorded Investment	48	78
Commercial Real Estate Automobile [Member]
Financing Receivable, Modifications [Line Items]
Financing Receivable, Modifications, Number of Contracts	0	5
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	20
Financing Receivable, Modifications, Post-Modification Recorded Investment	$ 0	$ 20

Total TDR movement during the period

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

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Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

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- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

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