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Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Noncash collateral placed with counterparties | $ 377 | $ 414
Cash collateral received from counterparties | $ 4 | $ 11

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- 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

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- 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

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