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

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

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