| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (As Restated) (Troubled Debt Restructuring Activity) (Details) $ in Thousands** | 3 Months Ended |           |
|-------------------------------------------------------------------------------------------------------------------------|----------------|-----------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Troubled debt restructurings                                                                                            | $ 3,238,900    |      | $ 2,385,175 |
| Number of Contracts | Contract                                                                                          | 80,142         | 10,472    |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 1,156,292    | $ 145,889 |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 1,150,986    | $ 143,232 |   |
| Corporate banking [Member]                                                                                              |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 2              | 2         |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 1,448        | $ 3,524   |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 1,439        | $ 3,505   |   |
| Middle market commercial real estate [Member]                                                                           |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 1              |      |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 14,439       |      |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 14,439       |      |   |
| Commercial and industrial [Member]                                                                                      |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 81             | 15        |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 10,872       | $ 18,859  |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 8,159        | $ 18,273  |   |
| Number of Contracts | Contract                                                                                          | 10             | 0         |   |
| Recorded Investment                                                                                                     | $ 276          | $ 0       |   |
| Other commercial [Member]                                                                                               |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          |           | 1         |   |
| Pre-Modification Outstanding Recorded Investment                                                                        |           | $ 498     |   |
| Post-Modification Outstanding Recorded Investment                                                                       |           | $ 500     |   |
| Residential mortgages [Member]                                                                                          |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 46             | 79        |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 7,866        | $ 17,811  |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 7,588        | $ 17,670  |   |
| Number of Contracts | Contract                                                                                          | 8              | 12        |   |
| Recorded Investment                                                                                                     | $ 931          | $ 1,915   |   |
| Home equity loans and lines of credit [Member]                                                                          |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 40             | 26        |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 3,671        | $ 2,285   |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 3,671        | $ 2,285   |   |
| Number of Contracts | Contract                                                                                          | 5              | 0         |   |
| Recorded Investment                                                                                                     | $ 262          | $ 0       |   |
| Retail installment contracts and auto loans [Member]                                                                    |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 9,693          | 40        |   |
| Recorded Investment                                                                                                     | $ 142,534      | $ 304     |   |
| Retail installment contracts and auto loans, originated [Member]                                                        |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 7,031          | 2         |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 137,751      | $ 54      |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 137,679      | $ 54      |   |
| Retail installment contracts and auto loans, purchased [Member]                                                         |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 68,458         | 10,346    |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 973,931      | $ 102,817 |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 971,743      | $ 100,903 |   |
| Personal unsecured loan [Member]                                                                                        |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 4,468          |      |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 5,394        |      |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 5,356        |      |   |
| Number of Contracts | Contract                                                                                          | 64             | 0         |   |
| Recorded Investment                                                                                                     | $ 58           | $ 0       |   |
| Other consumer [Member]                                                                                                 |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 15             | 1         |   |
| Pre-Modification Outstanding Recorded Investment                                                                        | $ 920          | $ 41      |   |
| Post-Modification Outstanding Recorded Investment                                                                       | $ 912          | $ 42      |   |
| Number of Contracts | Contract                                                                                          | 2              | 0         |   |
| Recorded Investment                                                                                                     | $ 244          | $ 0       |   |
| Consumer [Member]                                                                                                       |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Number of Contracts | Contract                                                                                          | 9,782          | 52        |   |
| Recorded Investment                                                                                                     | $ 144,305      | $ 2,219   |   |
| Performing [Member]                                                                                                     |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Troubled debt restructurings                                                                                            | 2,857,081      |      | 2,041,653 |
| Nonperforming [Member]                                                                                                  |           |      |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |           |      |   |
| Troubled debt restructurings                                                                                            | $ 381,819      |      | $ 343,522 |

| X                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in financing receivables (after modification) modified by troubled debt restructurings.
+ References
No definition available.
+ Details**Name:**sov_FinancingReceivableModificationsRecordedInvestmentPostModification**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in financing receivables (before modification) modified by troubled debt restructurings.
+ References
No definition available.
+ Details**Name:**sov_FinancingReceivableModificationsRecordedInvestmentPreModification**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modifications, Recorded Investment, Subsequent Default
+ References
No definition available.
+ Details**Name:**sov_FinancingReceivableModificationsRecordedInvestmentSubsequentDefault**Namespace Prefix:**sov_**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_FinancingReceivableModificationsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524
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| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_OtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=sov_PersonalUnsecuredLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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