| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (SCUSA Commitments) (Details) - USD ($)**           |             | 3 Months Ended | 9 Months Ended |               |               |               |               |             |
|--------------------------------------------------------------------------------------------------|-------------|----------------|----------------|---------------|---------------|---------------|---------------|-------------|
||
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Purchase commitment, repurchase rate (up to) (as a percent)                                      |        |           | 9.99%          |          |          |          |          |        |
| Purchase commitment, exercise of repurchase rights, retainer rate (up to) (as a percent)         |        |           | 20.00%         |          |          |          |          |        |
| Loan commitment, termination notice period                                                       |        |           | 120 days       |          |          |          |          |        |
| Loan purchase commitment, contract termination, effective period                                 |        |           | 90 days        |          |          |          |          |        |
| Guarantees, fair value                                                                           |        | $ 3,900,000    | $ 3,900,000    |          |          |          |          |        |
| Minimum sales commitment, charged off loan receivables                                           |        |           |           |          |          | $ 275,000,000 | $ 200,000,000 |        |
| Minimum sales commitment, loans receivable, written off, remaining                               |        | 203,400,000    | 203,400,000    |          |          |          |          |        |
| Mr. Dundon                                                                                       |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Severance-related expenses                                                                       |        | 12,300,000     |           |          |          |          |          |        |
| Stock compensation expense                                                                       |        | 9,900,000      |           |          |          |          |          |        |
| Contingent liability                                                                             |        | 115,100,000    | $ 115,100,000  |          |          |          |          |        |
| Civil fine | Violation of service members civil relief act                                       |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Civil fine, amount                                                                               | $ 55,000    |           |           |          |          |          |          |        |
| Lost equity for each repossession | Violation of service members civil relief act                |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Civil fine, amount                                                                               | 10,000      |           |           |          |          |          |          |        |
| Sought to collect repossession-related fees | Violation of service members civil relief act      |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Civil fine, amount                                                                               | 5,000       |           |           |          |          |          |          |        |
| Minimum | Civil fine to affected service members | Violation of service members civil relief act |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Civil fine, amount                                                                               | $ 9,400,000 |           |           |          |          |          |          |        |
| Maximum | Citizens Bank of Pennsylvania                                                          |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Loss-sharing payment percentage                                                                  |        |           | 0.50%          |          |          |          |          |        |
| Maximum | SCUSA                                                                                  |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Total commitments                                                                                |        | 350,000,000    | $ 350,000,000  |          |          |          |          |        |
| Loan purchase commitments | SCUSA                                                                |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Loan purchase commitment, cancellation period                                                    |        |           | 90 days        |          |          |          |          |        |
| Loan purchase commitments | Minimum | Citizens Bank of Pennsylvania                              |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Loan purchase commitment amount (lesser of)                                                      |        |           |           | $ 50,000,000  | $ 250,000,000 |          |          |        |
| Loan purchase commitments | Minimum | SCUSA                                                      |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Loan purchase commitment amount (lesser of)                                                      |        | $ 30,000,000   | $ 30,000,000   |          |          |          |          |        |
| Loan purchase commitment, as a percentage of lending entity's near-prime originations            |        | 50.00%         | 50.00%         |          |          |          |          |        |
| Loan purchase commitments | Maximum | Citizens Bank of Pennsylvania                              |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Loan purchase commitment amount (lesser of)                                                      |        |           |           | $ 200,000,000 | $ 600,000,000 |          |          |        |
| Forward commitments to sell loans | Obligation to repurchase receivables soldr] | SCUSA          |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Total commitments                                                                                |        | $ 15,700,000   | $ 15,700,000   |          |          |          |          | $ 7,700,000 |
| Forward commitments to sell loans | Maximum | SCUSA                                              |        |           |           |          |          |          |          |        |
| **Long-term Purchase Commitment [Line Items]**                                                   |        |           |           |          |          |          |          |        |
| Total commitments                                                                                |        | $ 300,000,000  | $ 300,000,000  |          |          |          |          |        |

| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed under commitments.
+ References
No definition available.
+ Details**Name:**sov_CommitmentsAmountCommitted**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Employee Benefits and Sharebased Compensation Contingent Liability
+ References
No definition available.
+ Details**Name:**sov_EmployeeBenefitsandSharebasedCompensationContingentLiability**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Commitment, Termination Notice Period
+ References
No definition available.
+ Details**Name:**sov_LoanCommitmentTerminationNoticePeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Amount
+ References
No definition available.
+ Details**Name:**sov_LoanPurchaseCommitmentAmount**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Cancellation Period
+ References
No definition available.
+ Details**Name:**sov_LoanPurchaseCommitmentCancellationPeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Contract Termination, Effective Period
+ References
No definition available.
+ Details**Name:**sov_LoanPurchaseCommitmentContractTerminationEffectivePeriod**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loan Purchase Commitment, Percentage of Lending Entity's Near-Prime Originations
+ References
No definition available.
+ Details**Name:**sov_LoanPurchaseCommitmentPercentageofLendingEntitysNearPrimeOriginations**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOff**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off, Remaining
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOffRemaining**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Profit (Loss) Sharing Payment, Percentage
+ References
No definition available.
+ Details**Name:**sov_ProfitLossSharingPaymentPercentage**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchase Commitment, Exercise of Repurchase Rights, Retainer Rate
+ References
No definition available.
+ Details**Name:**sov_PurchaseCommitmentExerciseofRepurchaseRightsRetainerRate**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Purchase Commitment, Repurchase Rate
+ References
No definition available.
+ Details**Name:**sov_PurchaseCommitmentRepurchaseRate**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F
+ Details**Name:**us-gaap_AllocatedShareBasedCompensationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of guarantee, including, but not limited to, financial standby letter of credit, fair value guarantee, guarantee of collection of scheduled contractual cash flows from financial assets, and contracts that contingently require the guarantor to make payments or perform services.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12201-110248
+ Details**Name:**us-gaap_GuaranteesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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_LongTermPurchaseCommitmentLineItems**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
+ Details**Name:**us-gaap_LossContingencyDamagesSoughtValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expenses for special or contractual termination benefits provided to current employees involuntarily terminated under a benefit arrangement associated exit or disposal activities pursuant to an authorized plan. Excludes expenses related to one-time termination benefits, a discontinued operation or an asset retirement obligation.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.P.4(b)) -URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.P.3) -URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140864-122747Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(10) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section P -Subsection 3, 4
+ Details**Name:**us-gaap_SeveranceCosts1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_TitleOfIndividualAxis=us-gaap_ChiefExecutiveOfficerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=sov_CivilFineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LitigationCaseAxis=sov_ViolationofServiceMembersCivilReliefActMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=sov_LostEquityforEachRepossessionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=sov_SoughttoCollectRepossessionrelatedFeesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=sov_CivilFinetoAffectedServiceMembersMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=sov_CitizensBankofPennsylvaniaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_ScusaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=us-gaap_LoanPurchaseCommitmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=sov_ForwardCommitmentsToSellLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_ObligationToRepurchaseReceivablesSoldMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |