v3.26.1
Derivative Instruments and Hedging Activities - Schedule of Interest and Amortization on Derivative Instruments (Details) - Total derivatives designated as accounting hedges - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain on fair value hedging relationships $ 6 $ 17 $ 11 $ 38
Income Statement Location [Axis]: us-gaap:InterestExpenseLongTermDebt        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain on fair value hedging relationships 3 3 5 5
Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain on fair value hedging relationships 6 19 10 41
Unsecured | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Unsecured | Income Statement Location [Axis]: us-gaap:InterestExpenseLongTermDebt        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 3 3 5 5
Unsecured | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Hedged available-for-sale securities | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Gain on interest for qualifying hedge 0 0 0 0
Hedged available-for-sale securities | Income Statement Location [Axis]: us-gaap:InterestExpenseLongTermDebt        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Gain on interest for qualifying hedge 0 0 0 0
Hedged available-for-sale securities | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 4 4 7 9
Gain on interest for qualifying hedge 2 15 3 32
Fixed-rate automotive loans | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 1 2 1 4
Gain on interest for qualifying hedge 5 15 10 34
Fixed-rate automotive loans | Income Statement Location [Axis]: us-gaap:InterestExpenseLongTermDebt        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Gain on interest for qualifying hedge 0 0 0 0
Fixed-rate automotive loans | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain on amortization of deferred basis adjustments 0 0 0 0
Gain on interest for qualifying hedge $ 0 $ 0 $ 0 $ 0