Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Pledged Assets related to secured borrowings [Line Items]
Pledged financial instruments, investment securities	$ 6,335	$ 8,371
Pledged assets, mortgage assets held-for-investment and lending receivables	16,299	13,579
Pledged assets,operating leases	213	457
Pledged assets, restricted as collateral	55,031	58,761
Secured debt	36,992	36,837
Ally Bank
Pledged Assets related to secured borrowings [Line Items]
Pledged financial instruments, investment securities	5,487	7,443
Pledged assets, mortgage assets held-for-investment and lending receivables	16,299	13,579
Pledged assets,operating leases	0	0
Pledged assets, restricted as collateral	46,456	43,694
Secured debt	29,118	23,278
Pledged assets for Federal Home Loan Bank \| Ally Bank
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, restricted as collateral	25,900	25,200
Pledged assets for Federal Reserve Bank \| Ally Bank
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, restricted as collateral	2,400	2,300
Short-term borrowings
Pledged Assets related to secured borrowings [Line Items]
Secured debt	4,800	8,200
Consumer portfolio segment \| Automobile loan
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, finance receivables	17,813	19,787
Consumer portfolio segment \| Automobile loan \| Ally Bank
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, finance receivables	10,333	6,200
Commercial portfolio segment \| Automobile loan
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, finance receivables	14,371	16,567
Commercial portfolio segment \| Automobile loan \| Ally Bank
Pledged Assets related to secured borrowings [Line Items]
Pledged assets, finance receivables	$ 14,337	$ 16,472

Pledged Assets, Investment in operating leases

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- Definition

Pledged Assets related to secured borrowings [Line Items]

+ References

No definition available.

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109249958&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://fasb.org/us-gaap/role/ref/legacyRef
-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=109249958&loc=SL6224234-111729

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- Definition

The fair value, as of the date of each statement of financial position presented, of mortgage-related securities, classified as available for sale or held for investment, 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 that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
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-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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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://fasb.org/us-gaap/role/ref/legacyRef
-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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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