| **FAIR VALUE (Fair Value Measurements, Non-recurring) (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                    |
|-------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Total carrying value of the loans                                                                                 | $ 3,495,289us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                                               | $ 2,629,038us-gaap_ImpairedFinancingReceivableRecordedInvestment |
| Nonrecurring [Member]                                                                                             |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Impaired loans held for investment                                                                                | 98,224sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                                                                                         |                                                                                                                                                                                                                                                                                               | 168,917sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Foreclosed assets                                                                                                 | 12,985sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                                                                                      |                                                                                                                                                                                                                                                                                               | 45,599sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Vehicle inventory                                                                                                 | 159,990sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                                                                                     |                                                                                                                                                                                                                                                                                               | 136,136sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Fair value adjustment                                                                                             | (1,181)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                                                                                           | (3,032)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                                                                                                             |   |
| Nonrecurring [Member] | Impaired loans held for investment [Member]                                               |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Total carrying value of the loans                                                                                 | 51,600us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_FairValueByAssetClassAxis= sov_ImpairedLoansHeldForInvestmentMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                           |                                                                                                                                                                                                                                                                                               | 100,200us-gaap_ImpairedFinancingReceivableRecordedInvestment/ us-gaap_FairValueByAssetClassAxis= sov_ImpairedLoansHeldForInvestmentMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Nonrecurring [Member] | Impaired loans held for investment [Member] | Provision for credit losses [Member]        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Fair value adjustment                                                                                             | (570)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByAssetClassAxis= sov_ImpairedLoansHeldForInvestmentMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember/ us-gaap_IncomeStatementLocationAxis= sov_ProvisionForLoanAndLeaseLossesMember | (2,809)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByAssetClassAxis= sov_ImpairedLoansHeldForInvestmentMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember/ us-gaap_IncomeStatementLocationAxis= sov_ProvisionForLoanAndLeaseLossesMember |   |
| Nonrecurring [Member] | Foreclosed assets [Member] | Other administrative expense [Member]                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Fair value adjustment                                                                                             | (611)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByAssetClassAxis= sov_ForeclosedAssetsMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember/ us-gaap_IncomeStatementLocationAxis= us-gaap_OtherExpenseMember                             | (223)us-gaap_AssetsFairValueAdjustment/ us-gaap_FairValueByAssetClassAxis= sov_ForeclosedAssetsMember/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember/ us-gaap_IncomeStatementLocationAxis= us-gaap_OtherExpenseMember                               |   |
| Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]                   |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Impaired loans held for investment                                                                                | 3,896sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                     |                                                                                                                                                                                                                                                                                               | 0sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Foreclosed assets                                                                                                 | 0sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                      |                                                                                                                                                                                                                                                                                               | 0sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Vehicle inventory                                                                                                 | 0sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                      |                                                                                                                                                                                                                                                                                               | 0sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]                                    |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Impaired loans held for investment                                                                                | 92,017sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                    |                                                                                                                                                                                                                                                                                               | 101,218sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Foreclosed assets                                                                                                 | 12,985sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                 |                                                                                                                                                                                                                                                                                               | 45,599sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Vehicle inventory                                                                                                 | 159,990sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                |                                                                                                                                                                                                                                                                                               | 136,136sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]                                        |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                  |                                                                                                                                                                                                                                                                                             |                                                                                                                                                                                                                                                                                               |   |
| Impaired loans held for investment                                                                                | 2,311sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                     |                                                                                                                                                                                                                                                                                               | 67,699sov_ImpairedLoansMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Foreclosed assets                                                                                                 | 0sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                      |                                                                                                                                                                                                                                                                                               | 0sov_ForeclosedAssetsMeasuredAtFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |
| Vehicle inventory                                                                                                 | $ 0sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember                                                                    |                                                                                                                                                                                                                                                                                               | $ 0sov_VehicleInventoryFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementFrequencyAxis= us-gaap_FairValueMeasurementsNonrecurringMember |

| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Foreclosed Assets Measured at Fair Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired Loans Measured at Fair Value
+ References
No definition available.
+ Details**Name:**sov_ImpairedLoansMeasuredAtFairValue**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Vehicle Inventory, Fair Value Disclosure
+ References
No definition available.
+ Details**Name:**sov_VehicleInventoryFairValueDisclosure**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of addition (reduction) to the amount at which an asset could be incurred (settled) in a current transaction between willing parties.
+ References
No definition available.
+ Details**Name:**us-gaap_AssetsFairValueAdjustment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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