Derivative Instruments and Hedging Activities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Noncash collateral placed with counterparties	$ 121	$ 118
Cash collateral received from counterparties	$ 5	40
Noncash collateral received from counterparties		$ 29

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