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Guarantees and Commitments (Guarantees to third parties) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Guarantor Obligations [Line Items]    
Standby letters of credit and other guarantees $ 19ally_StandbyLettersOfCreditAndOtherGuarantees $ 30ally_StandbyLettersOfCreditAndOtherGuarantees
Maximum [Member]    
Guarantor Obligations [Line Items]    
Standby letters of credit and other guarantees $ 268ally_StandbyLettersOfCreditAndOtherGuarantees
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 $ 142ally_StandbyLettersOfCreditAndOtherGuarantees
/ us-gaap_RangeAxis
= us-gaap_MaximumMember