| **INCOME TAXES (Details) (USD $)**                                                                             | 3 Months Ended                                                                                                    | 9 Months Ended                                           | 24 Months Ended                                                                                                   | 60 Months Ended                                          |
|----------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------|
||
| **Income Tax Disclosure [Abstract]**                                                                           |                                                                                                              |                                                     |                                                                                                              |                                                     |   |   |
| Income tax provision                                                                                           | $ 36,102,000us-gaap_IncomeTaxExpenseBenefit                                                                       | $ 18,692,000us-gaap_IncomeTaxExpenseBenefit              | $ 1,285,855,000us-gaap_IncomeTaxExpenseBenefit                                                                    | $ 156,670,000us-gaap_IncomeTaxExpenseBenefit             |   |   |
| Effective tax rate                                                                                             | 12.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations                                                          | 19.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 34.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations                                                          | 22.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations |   |   |
| Number of financing transactions related to lawsuit                                                            |                                                                                                              |                                                     |                                                                                                              |                                                     |   | 2sov_NumberofTransactionsRelatedtoLawsuitSeekingRefundofTaxesPaid |
| Transaction amount related to lawsuit seeking refund of taxes paid                                             |                                                                                                              |                                                     |                                                                                                              |                                                     |   | 1,200,000,000sov_TransactionAmountRelatedToLawsuitSeekingRefundOfTaxesPaid |
| Foreign taxes paid                                                                                             |                                                                                                              |                                                     |                                                                                                              |                                                     |   | 264,000,000us-gaap_IncomeTaxesPaid |
| Disallowed interest expense and transaction costs deductions                                                   |                                                                                                              |                                                     |                                                                                                              |                                                     |   | 74,600,000sov_DisallowedInterestExpenseAndTransactionCostsDeductions |
| Penalties and interest expense                                                                                 |                                                                                                              |                                                     |                                                                                                              |                                                     |   | 92,500,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense |
| Tax reserve                                                                                                    | 96,900,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued                                                 |                                                     | 96,900,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued                                                 |                                                     |   |   |
| Reasonably possible decrease in reserve for uncertain tax positions                                            | 96,900,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeLowerBound  |                                                     | 96,900,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeLowerBound  |                                                     |   |   |
| Reasonably possible increase in reserve for uncertain tax positions                                            | 296,000,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound |                                                     | 296,000,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound |                                                     |   |   |
| Maximum potential liability resulting from recharacteriazation of ordinary losses                              | 95,600,000sov_IncomeTaxExaminationRecharacterizationOfOrdinaryGains                                               |                                                     | 95,600,000sov_IncomeTaxExaminationRecharacterizationOfOrdinaryGains                                               |                                                     |   |   |
| Potential interest owed resulting from the recharacterization adjustment of ordinary losses to capital losses  |                                                                                                              |                                                     |                                                                                                              |                                                     | 11,100,000sov_PotentialInterestOwedResultingFromRecharacterizationAdjustmentOfOrdinaryLossesToCapitalLosses |   |
| Potential penalties owed resulting from the recharacterization adjustment of ordinary losses to capital losses |                                                                                                              |                                                     |                                                                                                              |                                                     | $ 14,500,000sov_PotentialPenaltiesOwedResultingFromRecharacterizationAdjustmentOfOrdinaryLossesToCapitalLosses |   |

| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Disallowed Interest Expense and Transaction Costs Deductions
+ References
No definition available.
+ Details**Name:**sov_DisallowedInterestExpenseAndTransactionCostsDeductions**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Examination Recharacterization of Ordinary Gains
+ References
No definition available.
+ Details**Name:**sov_IncomeTaxExaminationRecharacterizationOfOrdinaryGains**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Transactions Related to Lawsuit Seeking Refund of Taxes Paid
+ References
No definition available.
+ Details**Name:**sov_NumberofTransactionsRelatedtoLawsuitSeekingRefundofTaxesPaid**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Potential interest owed resulting from the recharacterization adjustment of ordinary losses to capital losses
+ References
No definition available.
+ Details**Name:**sov_PotentialInterestOwedResultingFromRecharacterizationAdjustmentOfOrdinaryLossesToCapitalLosses**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Potential penalties owed resulting from the recharacterization adjustment of ordinary losses to capital losses
+ References
No definition available.
+ Details**Name:**sov_PotentialPenaltiesOwedResultingFromRecharacterizationAdjustmentOfOrdinaryLossesToCapitalLosses**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Transaction Amount Related to Lawsuit Seeking Refund of Taxes Paid
+ References
No definition available.
+ Details**Name:**sov_TransactionAmountRelatedToLawsuitSeekingRefundOfTaxesPaid**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 2 -Article 4
+ Details**Name:**us-gaap_EffectiveIncomeTaxRateContinuingOperations**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IncomeTaxDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585
+ Details**Name:**us-gaap_IncomeTaxesPaid**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of estimated penalties and interest accrued as of the balance sheet date arising from income tax examinations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319
+ Details**Name:**us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The sum of the amounts of estimated penalties and interest recognized in the period arising from income tax examinations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319
+ Details**Name:**us-gaap_IncomeTaxExaminationPenaltiesAndInterestExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319
+ Details**Name:**us-gaap_IncomeTaxExpenseBenefit**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of (increase) decrease reasonably possible in the estimated lower bound of the unrecognized tax benefit for a tax position taken or expected to be taken.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319
+ Details**Name:**us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeLowerBound**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of (increase) decrease reasonably possible in the estimated upper bound of the unrecognized tax benefit for a tax position taken or expected to be taken.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319
+ Details**Name:**us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |