| **FAIR VALUE (Reconciliation of Assets and Liabilities Using Level 3 Inputs) (Details) $ in Thousands**   | 3 Months Ended |             |
|-----------------------------------------------------------------------------------------------------------|----------------|-------------|
||
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Number of primary factors driving gains | factor                                                          | 3              |        |   |
| RICs HFI                                                                                                  |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Unpaid principal balance on previously charged-off RIC portfolio                                          |           |        | $ 3,000,000 |
| Level 3 | Recurring                                                                                       |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Balances, beginning of period                                                                             | $ 684,230      | $ 1,149,326 |   |
| Losses in other comprehensive income                                                                      | (499)          | (619)       |   |
| Gains/(losses) in earnings                                                                                | 20,453         | 19,542      |   |
| Additions/Issuances                                                                                       | 4,104          | 19,061      |   |
| Settlements                                                                                               | (30,182)       | (289,637)   |   |
| Balances, end of period                                                                                   | 678,106        | 897,673     |   |
| Changes in unrealized gains (losses) included in earnings related to balances still held at end of period | 20,305         | 17,548      |   |
| Level 3 | Recurring | Investments AFS                                                                     |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Balances, beginning of period                                                                             | 350,252        | 814,567     |   |
| Losses in other comprehensive income                                                                      | (499)          | (619)       |   |
| Gains/(losses) in earnings                                                                                | 0              | 0           |   |
| Additions/Issuances                                                                                       | 0              | 0           |   |
| Settlements                                                                                               | (1,119)        | (240,494)   |   |
| Balances, end of period                                                                                   | 348,634        | 573,454     |   |
| Changes in unrealized gains (losses) included in earnings related to balances still held at end of period | 0              | 0           |   |
| Level 3 | Recurring | RICs HFI                                                                            |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Balances, beginning of period                                                                             | 186,471        | 217,170     |   |
| Losses in other comprehensive income                                                                      | 0              | 0           |   |
| Gains/(losses) in earnings                                                                                | 5,276          | 16,891      |   |
| Additions/Issuances                                                                                       | 1,349          | 13,331      |   |
| Settlements                                                                                               | (25,499)       | (44,919)    |   |
| Balances, end of period                                                                                   | 167,597        | 202,473     |   |
| Changes in unrealized gains (losses) included in earnings related to balances still held at end of period | 5,276          | 16,891      |   |
| Level 3 | Recurring | MSRs (11)                                                                           |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Balances, beginning of period                                                                             | 145,993        | 146,589     |   |
| Losses in other comprehensive income                                                                      | 0              | 0           |   |
| Gains/(losses) in earnings                                                                                | 15,043         | 1,457       |   |
| Additions/Issuances                                                                                       | 2,755          | 5,730       |   |
| Settlements                                                                                               | (3,661)        | (4,321)     |   |
| Balances, end of period                                                                                   | 160,130        | 149,455     |   |
| Changes in unrealized gains (losses) included in earnings related to balances still held at end of period | 15,043         | 1,457       |   |
| Level 3 | Recurring | Derivatives                                                                         |           |        |   |
| **Fair Value, Assets, Unobservable Input Reconciliation [Roll Forward]**                                  |           |        |   |
| Balances, beginning of period                                                                             | 1,514          | (29,000)    |   |
| Losses in other comprehensive income                                                                      | 0              | 0           |   |
| Gains/(losses) in earnings                                                                                | 134            | 1,194       |   |
| Additions/Issuances                                                                                       | 0              | 0           |   |
| Settlements                                                                                               | 97             | 97          |   |
| Balances, end of period                                                                                   | 1,745          | (27,709)    |   |
| Changes in unrealized gains (losses) included in earnings related to balances still held at end of period | $ (14)         | $ (800)     |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Number of Factors Driving Gain
+ References
No definition available.
+ Details**Name:**sov_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisNumberofFactorsDrivingGain**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired Financing Receivable, Previously Written-off, Unpaid Principal Balance
+ References
No definition available.
+ Details**Name:**sov_ImpairedFinancingReceivablePreviouslyWrittenoffUnpaidPrincipalBalance**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258
+ Details**Name:**us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInEarnings**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in other comprehensive income (loss) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258
+ Details**Name:**us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInOtherComprehensiveIncomeLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of issuances of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationIssues**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258
+ Details**Name:**us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationSettlements**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258
+ Details**Name:**us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=sov_RetailInstallmentContractsHeldforinvestmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_AvailableforsaleSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=sov_ServicingAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_DerivativeFinancialInstrumentsAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |