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Summary of Significant Accounting Policies - Narrative (Details) (USD $)
 1 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 1 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2014
payments
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2013
Oct. 31, 2014
Jan. 02, 2015
Dec. 31, 2012
Sep. 30, 2014
Jan. 03, 2015
Summary of Significant Accounting Policies [Line Items]                    
Contributions from GM $ 700,000,000us-gaap_ProceedsFromContributionsFromParent                  
Adjustment to net cash used by investing activities   148,000,000acf_AdjustmenttoNetcashUsedbyInvestingActivities                
Probable loss experience period   10 years                
Allowance for credit losses 31,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses 31,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses 31,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses 27,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses 27,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses          
Number of delinquent payments that can be moved   2acf_TroubledDebtRestructuringDebtorNumberofDelinquentPaymentsCustomerCanMove                
Intercompany subvention receivable 384,000,000acf_IntercompanySubventionReceivable 384,000,000acf_IntercompanySubventionReceivable 384,000,000acf_IntercompanySubventionReceivable 129,000,000acf_IntercompanySubventionReceivable 129,000,000acf_IntercompanySubventionReceivable          
Intercompany Receivable - Commercial Lending 176,000,000acf_IntercompanyReceivableCommercialLending 176,000,000acf_IntercompanyReceivableCommercialLending 176,000,000acf_IntercompanyReceivableCommercialLending 62,000,000acf_IntercompanyReceivableCommercialLending 62,000,000acf_IntercompanyReceivableCommercialLending          
Intercompany Receivables - Wholesale financing 289,000,000acf_IntercompanyReceivablesWholesalefinancing 289,000,000acf_IntercompanyReceivablesWholesalefinancing 289,000,000acf_IntercompanyReceivablesWholesalefinancing 588,000,000acf_IntercompanyReceivablesWholesalefinancing 588,000,000acf_IntercompanyReceivablesWholesalefinancing          
Intercompany Receivable - Commercial Receivables 433,000,000acf_IntercompanyReceivableCommercialReceivables 433,000,000acf_IntercompanyReceivableCommercialReceivables 433,000,000acf_IntercompanyReceivableCommercialReceivables 368,000,000acf_IntercompanyReceivableCommercialReceivables 368,000,000acf_IntercompanyReceivableCommercialReceivables          
Related party taxes payable 636,000,000acf_IntercompanyTaxesPayable 636,000,000acf_IntercompanyTaxesPayable 636,000,000acf_IntercompanyTaxesPayable 643,000,000acf_IntercompanyTaxesPayable 643,000,000acf_IntercompanyTaxesPayable          
Earnings asset leverage ratio 6.5acf_EarningAssetsLeverageRatio 6.5acf_EarningAssetsLeverageRatio 6.5acf_EarningAssetsLeverageRatio              
Total borrowing capacity 5,500,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity 5,500,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity 5,500,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity              
Line of Credit Facilities - GM Related Party Facility 600,000,000acf_LineOfCreditFacilitiesGmRelatedPartyFacility 600,000,000acf_LineOfCreditFacilitiesGmRelatedPartyFacility 600,000,000acf_LineOfCreditFacilitiesGmRelatedPartyFacility              
Minimum [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Property, Plant and Equipment, Useful Life   1 year                
Maximum [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Property, Plant and Equipment, Useful Life   25 years                
Earnings asset leverage ratio 8.0acf_EarningAssetsLeverageRatio
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 8.0acf_EarningAssetsLeverageRatio
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 8.0acf_EarningAssetsLeverageRatio
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		              
Ally Financial International Operations [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Payments to acquire business   3,300,000,000us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= acf_AllyFinancialInternationalOperationsMember		                
Liabilities incurred     1,400,000,000us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred
/ us-gaap_BusinessAcquisitionAxis
= acf_AllyFinancialInternationalOperationsMember		 1,400,000,000us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred
/ us-gaap_BusinessAcquisitionAxis
= acf_AllyFinancialInternationalOperationsMember		            
Aggregate consideration   3,300,000,000us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= acf_AllyFinancialInternationalOperationsMember		     3,300,000,000us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= acf_AllyFinancialInternationalOperationsMember		          
Consumer Finance Receivables [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Contractually delinquent period   120 days                
Commercial Finance Receivables [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Typical repayment period   90 days                
Contractually delinquent period   360 days                
Allowance for credit losses 40,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		 40,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		 40,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		 51,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		 51,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		     6,000,000us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialPortfolioSegmentMember		    
Commercial Finance Receivables [Member] | Minimum [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Dealer repayable period   2 days                
Credit facility expiration period   6 months                
Commercial Finance Receivables [Member] | Maximum [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Dealer repayable period   10 days                
Payment period after loan is called   30 days                
Credit facility expiration period   18 months                
Corporate Revolving Credit Facility [Member] | Line of Credit [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Total borrowing capacity           4,000,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= acf_CorporateRevolvingCreditFacilityMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		        
Junior Subordinated Revolving Credit Facility [Member] | Line of Credit [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Total borrowing capacity           1,000,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= acf_JuniorSubordinatedRevolvingCreditFacilityMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		        
Three and Five Year Credit Facilities [Member] | Line of Credit [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Total borrowing capacity           2,000,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= acf_ThreeandFiveYearCreditFacilitiesMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		        
Parent Company [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Number of revolving credit facilities           2acf_LineofCreditNumberofRevolvingCreditFacilities
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		        
Related party taxes payable 636,000,000acf_IntercompanyTaxesPayable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 636,000,000acf_IntercompanyTaxesPayable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 636,000,000acf_IntercompanyTaxesPayable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 643,000,000acf_IntercompanyTaxesPayable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 643,000,000acf_IntercompanyTaxesPayable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		          
Total borrowing capacity           12,500,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		     11,000,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		  
Parent Company [Member] | Three Year Credit Facility [Member] | Line of Credit [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Debt instrument term           3 years        
Total borrowing capacity           5,000,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= acf_ThreeYearCreditFacilityMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		        
Parent Company [Member] | Five Year Credit Facility [Member] | Line of Credit [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Debt instrument term           5 years        
Total borrowing capacity           7,500,000,000.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= acf_FiveYearCreditFacilityMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_LineOfCreditMember		        
Subsequent Event [Member] | SAIC GMAC [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Ownership percentage             40.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= acf_GmacSaicMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		     35.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= acf_GmacSaicMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Aggregate consideration             1,000,000,000us-gaap_BusinessCombinationConsiderationTransferred1
/ us-gaap_BusinessAcquisitionAxis
= acf_GmacSaicMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		      
Proceeds from sale of interest             $ 120,000,000us-gaap_ProceedsFromDivestitureOfBusinesses
/ us-gaap_BusinessAcquisitionAxis
= acf_GmacSaicMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		      
Subsequent Event [Member] | Subsidiaries [Member]                    
Summary of Significant Accounting Policies [Line Items]                    
Ownership percentage by noncontrolling owners (percent)             5.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember