| **LOANS AND ALLOWANCE FOR CREDIT LOSSES - Amortized Cost Basis Payment Defaults Prior Modification - Preceding 12 Months (Details) - USD ($) $ in Thousands** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | $ 131,564 | $ 411,144 |
| Payment Deferral Only                                                                                                                                         |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | $ 73,035        |   |   |
| All Other Modifications                                                                                                                                       |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 35,002          |   |   |
| Commercial: | CRE                                                                                                                                             |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 0 | 1,182 |
| Commercial: | CRE | Payment Deferral Only                                                                                                                     |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 7,144           |   |   |
| Commercial: | CRE | All Other Modifications                                                                                                                   |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 0               |   |   |
| Commercial: | C&I loans                                                                                                                                   |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 0 | 11,269 |
| Commercial: | C&I loans | Payment Deferral Only                                                                                                           |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 843             |   |   |
| Commercial: | C&I loans | All Other Modifications                                                                                                         |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 607             |   |   |
| Commercial: | Multifamily loans                                                                                                                               |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 0 | 0 |
| Commercial: | Multifamily loans | Payment Deferral Only                                                                                                       |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 8,536           |   |   |
| Commercial: | Multifamily loans | All Other Modifications                                                                                                     |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 0               |   |   |
| Commercial: | Other commercial                                                                                                                                |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 11 | 17 |
| Consumer: | Residential mortgages                                                                                                                             |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 28,945 | 33,689 |
| Consumer: | Residential mortgages | Payment Deferral Only                                                                                                     |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 0               |   |   |
| Consumer: | Residential mortgages | All Other Modifications                                                                                                   |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 507             |   |   |
| Consumer: | Home equity loans and lines of credit                                                                                                             |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 5,684 | 2,378 |
| Consumer: | Home equity loans and lines of credit | Payment Deferral Only                                                                                     |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 0               |   |   |
| Consumer: | Home equity loans and lines of credit | All Other Modifications                                                                                   |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 186             |   |   |
| Consumer: | RICs and auto loans                                                                                                                               |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 96,671 | 362,157 |
| Consumer: | RICs and auto loans | Payment Deferral Only                                                                                                       |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | 56,512          |   |   |
| Consumer: | RICs and auto loans | All Other Modifications                                                                                                     |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                | $ 33,702        |   |   |
| Consumer: | Personal unsecured loans                                                                                                                          |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | 196 | 254 |
| Consumer: | Other consumer                                                                                                                                    |            |   |   |
| **Financing Receivable, Modified [Line Items]**                                                                                                               |            |   |   |
| Amortized Cost                                                                                                                                                |            | $ 57 | $ 198 |

| 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
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 42 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-42Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 42 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-42Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 12A -Publisher FASB -URI https://asc.fasb.org//1943274/2147481933/310-10-55-12AReference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 33 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-33Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 34 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-34
+ Details**Name:**us-gaap_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 12A -Publisher FASB -URI https://asc.fasb.org//1943274/2147481933/310-10-55-12AReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 44 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-44Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 34 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481962/310-10-50-34
+ Details**Name:**us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LoanRestructuringModificationAxis=sov_AllOtherModificationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_CommercialRealEstateLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_MultiFamilyCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_OtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_RetailInstallmentContractsandAutoLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_PersonalUnsecuredLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_OtherConsumerLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |