v3.26.3
FAIR VALUE - Schedule of Increases and Decreases in Value of Certain Assets Measured at Fair Value on Nonrecurring Basis (Details) - Nonrecurring - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Impaired LHFI | Income Statement Location [Axis]: sov:CreditLossExpenseReversal        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value adjustment $ 23 $ (8) $ 38 $ (9)
Foreclosed assets | Income Statement Location [Axis]: us-gaap:OtherOperatingIncomeExpenseNet        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value adjustment 0 0 4 (11)
LHFS | Income Statement Location [Axis]: us-gaap:OtherOperatingIncomeExpenseNet        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value adjustment 0 0 (4) 0
LHFS | Income Statement Location [Axis]: sov:CreditLossExpenseReversal        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value adjustment (2) 0 (2) 9
Auto loans impaired due to bankruptcy | Income Statement Location [Axis]: sov:CreditLossExpenseReversal        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Fair value adjustment $ (8) $ 0 $ (7) $ 4