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Loans Held-for-Sale, Net Loans Held-for-Sale, Net (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loans held-for-sale, net $ 1,559us-gaap_LoansReceivableHeldForSaleNet $ 2,003us-gaap_LoansReceivableHeldForSaleNet
Consumer Loans Auto Financing Receivable [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loans held-for-sale, net 1,500us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= us-gaap_ConsumerLoansAutoFinancingReceivableMember		 1,515us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= us-gaap_ConsumerLoansAutoFinancingReceivableMember
Residential Mortgage [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loans held-for-sale, net 42us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= us-gaap_ResidentialMortgageMember		 452us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= us-gaap_ResidentialMortgageMember
Commercial And Industrial Other [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loans held-for-sale, net $ 17us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= ally_CommercialAndIndustrialOtherMember		 $ 36us-gaap_LoansReceivableHeldForSaleNet
/ us-gaap_ProductOrServiceAxis
= ally_CommercialAndIndustrialOtherMember