| **DERIVATIVES (As Restated) (Details) - USD ($) $ in Thousands**                                       | 3 Months Ended          | 12 Months Ended          |
|--------------------------------------------------------------------------------------------------------|-------------------------|--------------------------|
||
| **Derivative, Credit Risk Related Contingent Features [Abstract]**                                     |                    |                     |
| Fair value of derivatives with credit risk contingent feature associated with credit ratings           | $ 23,300                |                     |
| Fair value of derivatives with credit risk contingent features                                         | 157,300                 | $ 133,200                |
| Collateral posted                                                                                      | 145,100                 | 127,600                  |
| **Summary of Cash Flow Hedge Activity [Abstract]**                                                     |                    |                     |
| Cash flow hedge gain (loss) to be reclassified within next twelve months                               | (5,800)                 |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | 42,385,338              | 40,020,177               |
| Interest rate swaps [Member]                                                                           |                    |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | 2,939,000               | 3,231,000                |
| Designated as hedging instrument [Member]                                                              |                    |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | 9,737,182               | 10,361,333               |
| Total, Asset                                                                                           | 4,735                   | 10,562                   |
| Total, Liability                                                                                       | $ 42,608                | $ 22,311                 |
| Weighted Average Receive Rate                                                                          | 0.21%                   | 0.19%                    |
| Weighted Average Pay Rate                                                                              | 0.43%                   | 1.14%                    |
| Weighted Average Life (Years)                                                                          | 3 years 1 month 14 days | 3 years 18 days          |
| Designated as hedging instrument [Member] | Fair value hedges [Member] | Cross currency swaps [Member] |                    |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | $ 16,118                | $ 18,230                 |
| Fair value hedges, Asset                                                                               | 4,624                   | 2,711                    |
| Fair value hedges, Liability                                                                           | $ 700                   | $ 980                    |
| Weighted Average Receive Rate                                                                          | 4.76%                   | 4.76%                    |
| Weighted Average Pay Rate                                                                              | 4.75%                   | 4.75%                    |
| Weighted Average Life (Years)                                                                          | 10 months 9 days        | 1 year 1 month 9 days    |
| Designated as hedging instrument [Member] | Fair value hedges [Member] | Interest rate swaps [Member]  |                    |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | $ 291,000               | $ 257,000                |
| Fair value hedges, Asset                                                                               | 0                       | 232                      |
| Fair value hedges, Liability                                                                           | $ 3,539                 | $ 779                    |
| Weighted Average Receive Rate                                                                          | 0.93%                   | 0.90%                    |
| Weighted Average Pay Rate                                                                              | 2.37%                   | 2.38%                    |
| Weighted Average Life (Years)                                                                          | 4 years 2 months 3 days | 4 years 3 months 29 days |
| Designated as hedging instrument [Member] | Cash flow hedges [Member] | Interest rate swaps [Member]   |                    |                     |
| **Summary of Derivative Instruments [Abstract]**                                                       |                    |                     |
| Notional Amount                                                                                        | $ 9,430,064             | $ 10,086,103             |
| Cash flow hedges, Asset                                                                                | 111                     | 7,619                    |
| Cash flow hedges, Liability                                                                            | $ 38,369                | $ 20,552                 |
| Weighted Average Receive Rate                                                                          | 0.18%                   | 0.17%                    |
| Weighted Average Pay Rate                                                                              | 0.36%                   | 1.11%                    |
| Weighted Average Life (Years)                                                                          | 3 years 1 month 1 day   | 3 years 8 days           |
| One notch downgrade [Member]                                                                           |                    |                     |
| **Derivative, Credit Risk Related Contingent Features [Abstract]**                                     |                    |                     |
| Additional collateral required                                                                         | $ 3,600                 |                     |
| Two notch downgrade [Member]                                                                           |                    |                     |
| **Derivative, Credit Risk Related Contingent Features [Abstract]**                                     |                    |                     |
| Additional collateral required                                                                         | $ 5,600                 |                     |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B
+ Details**Name:**invest_DerivativeNotionalAmount**Namespace Prefix:**invest_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of derivatives with contingent features affected by the downgrade of credit rating of the entity.
+ References
No definition available.
+ Details**Name:**sov_DerivativeNetLiabilityPositionCreditDowngradeAggregateFairValue**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624258-113959
+ Details**Name:**us-gaap_AdditionalCollateralAggregateFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994
+ Details**Name:**us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624258-113959
+ Details**Name:**us-gaap_CollateralAlreadyPostedAggregateFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average fixed interest rate related to the group of interest rate derivatives.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeAverageFixedInterestRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeAverageRemainingMaturity1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average variable interest rate related to the group of interest rate derivatives.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeAverageVariableInterestRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_DerivativeCreditRiskRelatedContingentFeaturesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate fair value of all derivative assets designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611
+ Details**Name:**us-gaap_DerivativeInstrumentsInHedgesAssetsAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate fair value of all derivative liabilities designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611
+ Details**Name:**us-gaap_DerivativeInstrumentsInHedgesLiabilitiesAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624258-113959
+ Details**Name:**us-gaap_DerivativeNetLiabilityPositionAggregateFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of all derivative assets designated as foreign currency fair value hedging instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
+ Details**Name:**us-gaap_ForeignCurrencyFairValueHedgeAssetAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of all derivative liabilities designated as foreign currency fair value hedging instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
+ Details**Name:**us-gaap_ForeignCurrencyFairValueHedgeLiabilityAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of all interest rate derivative assets designated as cash flow hedging instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
+ Details**Name:**us-gaap_InterestRateCashFlowHedgeAssetAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
+ Details**Name:**us-gaap_InterestRateCashFlowHedgeLiabilityAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_SummaryOfCashFlowHedgeActivityAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_SummaryOfDerivativeInstrumentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember**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_CurrencySwapMember**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:**sov_CreditDowngradeLevelAxis=sov_OneNotchDowngradeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CreditDowngradeLevelAxis=sov_TwoNotchDowngradeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |