| **DERIVATIVES - Gains (Losses) on All Derivatives (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|----------------------------------------------------------------------------------------|----------------|
||
| Cross-currency swaps | Fair value hedges                                               |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | $ (830)        | $ (367) |
| Interest rate swaps | Fair value hedges                                                |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | 5,400          | 7,088 |
| Pay fixed-receive variable interest rate swaps | Cash flow hedges                      |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | (3,468)        | (7,275) |
| Pay variable receive-fixed interest rate swap | Cash flow hedges                       |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | (101,917)      | 23,251 |
| Interest rate floors | Cash flow hedges                                                |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | (436)          | 67 |
| Mortgage Banking derivatives                                                           |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | 2,383          | (18,823) |
| Customer-related derivatives                                                           |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | 17,057         | 4,525 |
| Foreign exchange                                                                       |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | (10,281)       | 1,962 |
| Interest rate swaps, caps, and options                                                 |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | (694)          | (22) |
| Other                                                                                  |           |   |
| **Derivative Instruments, Gain (Loss) [Line Items]**                                   |           |   |
| Gain (loss) recognized on derivatives                                                  | $ (57,558)     | $ 371 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4A
+ Details**Name:**us-gaap_DerivativeGainLossOnDerivativeNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4E -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4E
+ Details**Name:**us-gaap_DerivativeInstrumentsGainLossLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CurrencySwapMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=sov_InterestRateSwapsPayFixedReceiveVariableMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=sov_InterestRateSwapsPayVariableReceiveFixedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateFloorMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=sov_MortgageBankingDerivativesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=sov_CustomerRelatedDerivativesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=sov_SCDerivativesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |