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Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]                      
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate                 $ 436us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ 125us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate $ 180us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Income Tax Reconciliation, Change in Deferred Tax Assets Valuation Allowance                 (64)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance (154)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance (1,022)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Effective Income Tax Rate Reconciliation, Change in Unrecognized Tax Benefits                 (63)ally_EffectiveIncomeTaxRateReconciliationChangeinUnrecognizedTaxBenefits (10)ally_EffectiveIncomeTaxRateReconciliationChangeinUnrecognizedTaxBenefits (6)ally_EffectiveIncomeTaxRateReconciliationChangeinUnrecognizedTaxBenefits
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount                 (39)us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate (44)us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate 0us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate
Income Tax Reconciliation, Foreign Capital Loss                 (10)ally_IncomeTaxReconciliationForeignCapitalLoss (45)ally_IncomeTaxReconciliationForeignCapitalLoss (45)ally_IncomeTaxReconciliationForeignCapitalLoss
Income Tax Reconciliation, Equity in Earnings (Losses) of Unconsolidated Subsidiary                 31us-gaap_IncomeTaxReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary 26us-gaap_IncomeTaxReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary 12us-gaap_IncomeTaxReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary
Income Tax Reconciliation, State and Local Income Taxes                 48us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 16us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes (34)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Income Tax Reconciliation, Other Adjustments                 (18)us-gaap_IncomeTaxReconciliationOtherAdjustments 27us-gaap_IncomeTaxReconciliationOtherAdjustments 59us-gaap_IncomeTaxReconciliationOtherAdjustments
Income tax expense (benefit) from continuing operations $ 36us-gaap_IncomeTaxExpenseBenefit $ 127us-gaap_IncomeTaxExpenseBenefit $ 64us-gaap_IncomeTaxExpenseBenefit $ 94us-gaap_IncomeTaxExpenseBenefit $ (4)us-gaap_IncomeTaxExpenseBenefit $ 28us-gaap_IncomeTaxExpenseBenefit $ 40us-gaap_IncomeTaxExpenseBenefit $ (123)us-gaap_IncomeTaxExpenseBenefit $ 321us-gaap_IncomeTaxExpenseBenefit $ (59)us-gaap_IncomeTaxExpenseBenefit $ (856)us-gaap_IncomeTaxExpenseBenefit