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Derivative Instruments And Hedging Activities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Forward Contract Indexed to Issuer's Equity [Line Items]    
Cash and securities collateral placed with counterparties $ 228ally_CashAndSecuritiesCollateralPlacedWithCounterparties $ 236ally_CashAndSecuritiesCollateralPlacedWithCounterparties
Non-derivative collateral placed with counterparties 18ally_Nonderivativecollateralplacedwithcounterparties 18ally_Nonderivativecollateralplacedwithcounterparties
Cash collateral received from counterparties 118ally_CashCollateralReceivedFromCounterparties 71ally_CashCollateralReceivedFromCounterparties
Non-derivative cash collateral received from counterparties 13ally_Nonderivativecashcollateralreceivedfromcounterparties  
noncash collateral received 24ally_NoncashCollateralReceived 15ally_NoncashCollateralReceived
Other Assets [Member]    
Forward Contract Indexed to Issuer's Equity [Line Items]    
Prepaid Expense $ 62us-gaap_PrepaidExpenseCurrentAndNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember