v3.26.1
Derivative Instruments and Hedging Activities - Schedule of Derivative Instruments Designated as Fair Value Hedges, Gain (Loss) (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Derivative Instruments, Gain (Loss) [Line Items]        
Interest and fees on finance receivables and loans $ 2,741,000,000 $ 2,624,000,000 $ 5,399,000,000 $ 5,333,000,000
Interest and dividends on investment securities and other earning assets 242,000,000 248,000,000 476,000,000 478,000,000
Interest on long-term debt 266,000,000 258,000,000 531,000,000 529,000,000
Gain (loss) on cash flow hedges to be recognized within twelve months     0  
Total derivatives designated as accounting hedges | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain on fair value hedging relationships 0 0 0 0
Total loss on cash flow hedging relationships (4,000,000) (8,000,000) (9,000,000) (15,000,000)
Total derivatives designated as accounting hedges | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Total gain on fair value hedging relationships 0 0 0 0
Total loss on cash flow hedging relationships 0 0 0 0
Total derivatives designated as accounting hedges | Hedged available-for-sale securities | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in unrealized (loss) gain on hedged item in fair value hedge 0 0 0 0
Change in unrealized (loss) gain on fair value hedging instruments 0 0 0 0
Total derivatives designated as accounting hedges | Hedged available-for-sale securities | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in unrealized (loss) gain on hedged item in fair value hedge (92,000,000) 84,000,000 (169,000,000) 214,000,000
Change in unrealized (loss) gain on fair value hedging instruments 92,000,000 (84,000,000) 169,000,000 (214,000,000)
Total derivatives designated as accounting hedges | Fixed-rate automotive loans | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in unrealized (loss) gain on hedged item in fair value hedge (11,000,000) 12,000,000 (41,000,000) 41,000,000
Change in unrealized (loss) gain on fair value hedging instruments 11,000,000 (12,000,000) 41,000,000 (41,000,000)
Total derivatives designated as accounting hedges | Fixed-rate automotive loans | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in unrealized (loss) gain on hedged item in fair value hedge 0 0 0 0
Change in unrealized (loss) gain on fair value hedging instruments 0 0 0 0
Total derivatives designated as accounting hedges | Hedged variable-rate commercial loans | Income Statement Location [Axis]: us-gaap:InterestAndFeeIncomeLoansAndLeasesHeldInPortfolio        
Derivative Instruments, Gain (Loss) [Line Items]        
Interest rate cash flow hedge loss reclassified to earnings (4,000,000) (8,000,000) (9,000,000) (15,000,000)
Total derivatives designated as accounting hedges | Hedged variable-rate commercial loans | Income Statement Location [Axis]: ally:InterestAndDividendIncomeSecuritiesAndOtherEarningAssetsOperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Interest rate cash flow hedge loss reclassified to earnings $ 0 $ 0 $ 0 $ 0