| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Litigation) (Details) $ in Millions** |        | 6 Months Ended | 12 Months Ended |        |        |        |        |
|-------------------------------------------------------------------------------------|--------|----------------|-----------------|--------|--------|--------|--------|
||
| **Loss Contingencies [Line Items]**                                                 |   |           |            |   |   |   |   |
| Loss contingency, maximum possible loss                                             |   | $ 3.6          |            |   |   |   |   |
| Foreclosure matters [Member]                                                        |   |           |            |   |   |   |   |
| **Loss Contingencies [Line Items]**                                                 |   |           |            |   |   |   |   |
| Consent order, number of mortgage servicers | mortgage_servicer                     |   |           |            |   |   |   | 14     |
| Number of mortgage servicing companies | mortgage_servicer                          |   |           |            | 10     | 12     | 9      |   |
| Remediation fund amount                                                             |   | $ 6.2          |            |   |   |   |   |
| Remediation fund engaging period                                                    |   | 2 years        |            |   |   |   |   |
| Principal engaging in mortgage modifications                                        |   | $ 9.9          |            |   |   |   |   |
| Mortgage loan amount submitted for credit from OCC                                  |   |           | $ 74.1          |   |   |   |   |
| Litigation settlement incurred by the Bank as percentage of total settlement amount |   | 0.17%          |            |   |   |   |   |
| Litigation settlement amount accumulated by different banks                         |   | $ 9,300.0      |            |   |   |   |   |
| Litigation settlement gross amount                                                  |   | 16.1           |            |   |   |   |   |
| Identity theft protection product matter [Member]                                   |   |           |            |   |   |   |   |
| **Loss Contingencies [Line Items]**                                                 |   |           |            |   |   |   |   |
| Identity theft protection payments returned to customers                            | $ 6.0  | 37.6           |            |   |   |   |   |
| Pending gain (loss) related to litigation settlement                                |   | $ (4.7)        |            |   |   |   |   |

| X                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Consent Order, Mortgage servicing provider
+ References
No definition available.
+ Details**Name:**sov_ConsentOrderMortgageServicers**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gain (Loss) Related to Litigation Settlement, Pending
+ References
No definition available.
+ Details**Name:**sov_GainLossRelatedtoLitigationSettlementPending**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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 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.
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| 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                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of litigation expense, including but not limited to legal, forensic, accounting, and investigative fees.
+ References
No definition available.
+ Details**Name:**us-gaap_LitigationSettlementExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The value (monetary amount) of the award the plaintiff seeks in the legal matter.
+ 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=d3e14326-108349Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349
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| 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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| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LitigationCaseAxis=sov_ForeclosureMattersMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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