| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Litigation) (Details) (USD $)**                   | 6 Months Ended | 12 Months Ended |        |        |        |        |
|-------------------------------------------------------------------------------------------------|----------------|-----------------|--------|--------|--------|--------|
||
| **Loss Contingencies [Line Items]**                                                             |           |            |   |   |   |   |
| Loss contingency, maximum possible loss                                                         | $ 11,700,000   |            |   |   |   |   |
| Foreclosure Matters [Member]                                                                    |           |            |   |   |   |   |
| **Loss Contingencies [Line Items]**                                                             |           |            |   |   |   |   |
| Consent order, number of mortgage servicers                                                     |           |            |   |   |   | 14     |
| Number of mortgage servicing companies                                                          |           |            | 10     | 12     | 9      |   |
| Remediation fund amount                                                                         | 6,200,000      |            |   |   |   |   |
| Remediation fund engaging period                                                                | 2 years        |            |   |   |   |   |
| Principal engaging in mortgage modifications                                                    | 9,900,000      |            |   |   |   |   |
| Mortgage loan amount submitted for credit from OCC                                              |           | 74,100,000      |   |   |   |   |
| Litigation settlement incurred by the Bank as percentage of total settlement amount             | 0.17%          |            |   |   |   |   |
| Litigation settlement amount accumulated by different banks                                     | 9,300,000,000  |            |   |   |   |   |
| Litigation settlement gross amount                                                              | 16,100,000     |            |   |   |   |   |
| Identity Theft Protection Product Matter [Member]                                               |           |            |   |   |   |   |
| **Loss Contingencies [Line Items]**                                                             |           |            |   |   |   |   |
| Identity theft protection payments returned to customers                                        | 13,200,000     |            |   |   |   |   |
| Identity Theft Protection Product Matter [Member] | General and Administrative Expense [Member] |           |            |   |   |   |   |
| **Loss Contingencies [Line Items]**                                                             |           |            |   |   |   |   |
| Expected fees to be returned to customers                                                       |           | $ 30,000,000    |   |   |   |   |

| X                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Consent Order, Mortgage servicing provider
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Expected Fees to be Returned to Customers
+ References
No definition available.
+ Details**Name:**sov_ExpectedFeestobeReturnedtoCustomers**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Litigation Settlement, Amount Accumulated by Different Banks
+ References
No definition available.
+ Details**Name:**sov_LitigationSettlementAmountAccumulatedbyDifferentBanks**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Litigation Settlement, Expenses, Principal Engaging in Mortgage Modifications
+ References
No definition available.
+ Details**Name:**sov_LitigationSettlementExpensesPrincipalEngaginginMortgageModifications**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Litigation Settlement, Expenses, Remediation Fund
+ References
No definition available.
+ Details**Name:**sov_LitigationSettlementExpensesRemediationFund**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Litigation Settlement Incurred by Entity as Percentage of Total Settlement Amount
+ References
No definition available.
+ Details**Name:**sov_LitigationSettlementIncurredByEntityAsPercentageOfTotalSettlementAmount**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Litigation Settlement, Remediation Fund Engaging Period
+ References
No definition available.
+ Details**Name:**sov_LitigationSettlementRemediationFundEngagingPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortgage Loan Amount Submitted for Credit
+ References
No definition available.
+ Details**Name:**sov_MortgageLoanAmountSubmittedforCredit**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Mortgage Servicing Companies
+ References
No definition available.
+ Details**Name:**sov_NumberofMortgageServicingCompanies**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349
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| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of judgment or settlement awarded to (against) the entity in respect of litigation.
+ References
No definition available.
+ Details**Name:**us-gaap_LitigationSettlementAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The estimated maximum amount of possible loss from the contingency.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349
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