Insurance Premiums and Service Revenue Earned (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums Written and Earned [Abstract]
Direct Premiums Written, Property and Casualty	$ 294us-gaap_DirectPremiumsWritten	$ 270us-gaap_DirectPremiumsWritten	$ 332us-gaap_DirectPremiumsWritten
Direct Premiums Earned, Property and Casualty	282us-gaap_DirectPremiumsEarned	305us-gaap_DirectPremiumsEarned	335us-gaap_DirectPremiumsEarned
Assumed Premiums Written, Property and Casualty	43us-gaap_AssumedPremiumsWritten	61us-gaap_AssumedPremiumsWritten	44us-gaap_AssumedPremiumsWritten
Assumed Premiums Earned, Property and Casualty	54us-gaap_AssumedPremiumsEarned	58us-gaap_AssumedPremiumsEarned	49us-gaap_AssumedPremiumsEarned
Premiums Written, Gross	337us-gaap_PremiumsWrittenGross	331us-gaap_PremiumsWrittenGross	376us-gaap_PremiumsWrittenGross
Gross Insurance Premiums Earned	336ally_GrossInsurancePremiumsEarned	363ally_GrossInsurancePremiumsEarned	384ally_GrossInsurancePremiumsEarned
Ceded Premiums Written, Property and Casualty	(156)us-gaap_CededPremiumsWritten	(172)us-gaap_CededPremiumsWritten	(141)us-gaap_CededPremiumsWritten
Ceded Premiums Earned, Property and Casualty	(117)us-gaap_CededPremiumsEarned	(120)us-gaap_CededPremiumsEarned	(109)us-gaap_CededPremiumsEarned
Premiums Written, Net, Property and Casualty	181us-gaap_PremiumsWrittenNet	159us-gaap_PremiumsWrittenNet	235us-gaap_PremiumsWrittenNet
Premiums Earned, Net, Property and Casualty	219	243	275
Insurance Commissions and Fees Written	842ally_InsuranceCommissionsAndFeesWritten	838ally_InsuranceCommissionsAndFeesWritten	826ally_InsuranceCommissionsAndFeesWritten
Insurance Commissions and Fees	760us-gaap_InsuranceCommissionsAndFees	769us-gaap_InsuranceCommissionsAndFees	780us-gaap_InsuranceCommissionsAndFees
Insurance Services Revenue Written	1,023ally_InsuranceServicesRevenueWritten	997ally_InsuranceServicesRevenueWritten	1,061ally_InsuranceServicesRevenueWritten
Insurance premiums and service revenue earned	$ 979us-gaap_InsuranceServicesRevenue	$ 1,012us-gaap_InsuranceServicesRevenue	$ 1,055us-gaap_InsuranceServicesRevenue

Gross Insurance Premiums Earned

No definition available.

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- Definition

Insurance Commissions and Fees Written

+ References

No definition available.

+ Details

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- Definition

Insurance Services Revenue Written

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

Amount of earned premiums assumed from other entities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Amount of written premiums assumed from other entities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b)(2),(c)(2)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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Balance Type:	credit
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- Definition

Amount of earned premiums ceded to other entities.

+ References

+ Details

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

Amount of written premiums ceded to other entities.

+ References

+ Details

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

- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 17

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.12-17)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)(1)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.4)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(c),(d))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Subparagraph c, d

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 4

-Article 7

+ Details

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- Definition

Revenue from insurance services, including net premiums earned, gain on sale of insurance block, agency management fees and insurance contract fees and commissions.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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- Definition

Amount of direct and assumed premiums written.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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- Definition

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

+ Details

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