Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash collateral placed with counterparties	$ 1
Noncash collateral placed with counterparties	409	$ 414
Cash collateral received from counterparties	$ 2	$ 11

Cash Collateral Received From Counterparties, Excluding Collateral Associated With Repurchase Agreements

No definition available.

Name:	ally_CashCollateralReceivedFromCounterpartiesExcludingCollateralAssociatedWithRepurchaseAgreements
Namespace Prefix:	ally_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant