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Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Cash and securities collateral placed with counterparties | $ 2 |
Noncash collateral placed with counterparties | 143 | $ 118
Cash collateral received from counterparties | $ 2 | 40
Noncash collateral received from counterparties |  | $ 29

X
- Definition The amount of the right to reclaim collateral under legally enforceable agreements that enable the posting and receiving of collateral associated with the fair value of our derivative positions on an ongoing basis. + References No definition available. + Details
Name: | ally_CashAndSecuritiesCollateralPlacedWithCounterparties
Namespace Prefix: | ally_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cash Collateral Received from Counterparties, Excluding Collateral Associated with Repurchase Agreements + References No definition available. + Details
Name: | ally_CashCollateralReceivedfromCounterpartiesExcludingCollateralAssociatedwithRepurchaseAgreements
Namespace Prefix: | ally_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Non-cash collateral received from Counterparties + References No definition available. + Details
Name: | ally_NoncashcollateralreceivedfromCounterparties
Namespace Prefix: | ally_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Securities Collateral Placed with Counterparties + References No definition available. + Details
Name: | ally_SecuritiesCollateralPlacedwithCounterparties
Namespace Prefix: | ally_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration