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Guarantees and Commitments (Contractual financing commitments) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Guarantees [Abstract]    
Commitment to provide capital to investees $ 66ally_CommitmentToProvideCapitalToInvestees $ 63ally_CommitmentToProvideCapitalToInvestees
Warehouse and construction lending commitments 110ally_WarehouseAndConstructionLendingCommitments 187ally_WarehouseAndConstructionLendingCommitments
Commitment to fund remaining unused balances on home equity lines of credit 371ally_Commitmenttofundremainingunusedbalancesonhomeequitylinesofcredit 388ally_Commitmenttofundremainingunusedbalancesonhomeequitylinesofcredit
Unused revolving credit line commitments and other $ 1,284ally_UnusedRevolvingCreditLineCommitments $ 1,062ally_UnusedRevolvingCreditLineCommitments