Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Pledged Assets related to secured borrowings [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	$ 8,891	$ 8,371
Pledged Assets, Mortgage assets held for investment	14,501	13,579
Pledged Assets, Investment in operating leases	350	457
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	55,832	58,761
Secured Debt	36,659	36,837
Short-term Debt	9,564	11,413
Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	8,443	7,443
Pledged Assets, Mortgage assets held for investment	14,501	13,579
Pledged Assets, Investment in operating leases	0	0
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	44,141	43,694
Secured Debt	25,874	23,278
Pledged assets for Federal Home Loan Bank [Member] \| Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	27,000	25,200
Pledged assets for Federal Reserve Bank [Member] \| Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	2,300	2,300
Consumer Portfolio Segment [Member] \| Automobile Loan [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	17,472	19,787
Consumer Portfolio Segment [Member] \| Automobile Loan [Member] \| Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	6,639	6,200
Commercial Portfolio Segment [Member] \| Automobile Loan [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	14,618	16,567
Commercial Portfolio Segment [Member] \| Automobile Loan [Member] \| Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	$ 14,558	$ 16,472

Pledged Assets, Investment in operating leases

No definition available.

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- Definition

Pledged Assets, Mortgage assets held for investment

+ References

No definition available.

+ Details

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- Definition

Pledged Assets related to secured borrowings [Line Items]

+ References

No definition available.

+ Details

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of finance receivables which are owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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- Definition

The carrying amount, as of the date of the latest financial statement presented, of all assets that are pledged as collateral for the payment of the related debt obligation primarily arising from secured borrowings, repurchase agreements or other current or potential obligations, where the secured party does not have the right by contract or custom to sell or re-pledge the collateral and therefore are not reclassified and separately reported in the statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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- Definition

The carrying amount as of the date of the latest financial statement presented of other securities classified as available for sale or held for investment which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

+ Details

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- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: 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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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- 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

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- Details

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- Details

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