Derivative Instruments and Hedging Activities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash collateral placed with counterparties	$ 7	$ 4
Noncash collateral placed with counterparties	258	$ 145
Cash collateral received from counterparties	$ 8

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