| **STOCKHOLDER'S EQUITY (Details) (USD $)In Thousands, except Share data, unless otherwise specified** | 12 Months Ended                                                                                        | 1 Months Ended                                                                                 | 0 Months Ended                                                                                 | 3 Months Ended                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| **Class of Stock [Line Items]**                                                                       |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| Proceeds from the issuance of common stock                                                            | $ 1,787,533us-gaap_ProceedsFromIssuanceOfCommonStock                                                   | $ 0us-gaap_ProceedsFromIssuanceOfCommonStock                                                   | $ 0us-gaap_ProceedsFromIssuanceOfCommonStock                                                   |                                                                                                                                                               |   |   |   |
| Common Stock, shares outstanding                                                                      | 530,391,043us-gaap_CommonStockSharesOutstanding                                                        | 520,307,043us-gaap_CommonStockSharesOutstanding                                                |                                                                                           |                                                                                                                                                               |   | 530,391,043us-gaap_CommonStockSharesOutstanding |   |
| Santander [Member] | Stock Issued for Cash [Member]                                                   |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| **Class of Stock [Line Items]**                                                                       |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| Issuance of common stock, shares                                                                      |                                                                                                   |                                                                                           |                                                                                           | 7,000,000us-gaap_StockIssuedDuringPeriodSharesNewIssues/ dei_LegalEntityAxis= sov_SantanderMember/ us-gaap_SubsidiarySaleOfStockAxis= sov_StockIssuedforCashMember |   |   |   |
| Proceeds from the issuance of common stock                                                            |                                                                                                   |                                                                                           |                                                                                           | 1,750,000us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= sov_SantanderMember/ us-gaap_SubsidiarySaleOfStockAxis= sov_StockIssuedforCashMember      |   |   |   |
| Santander [Member] | Extinguishment of Debt [Member]                                                  |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| **Class of Stock [Line Items]**                                                                       |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| Issuance of common stock, shares                                                                      |                                                                                                   |                                                                                           |                                                                                           | 3,000,000us-gaap_StockIssuedDuringPeriodSharesNewIssues/ dei_LegalEntityAxis= sov_SantanderMember/ us-gaap_SubsidiarySaleOfStockAxis= us-gaap_LongTermDebtMember   |   |   |   |
| Proceeds from the issuance of common stock                                                            |                                                                                                   |                                                                                           |                                                                                           | 750,000us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= sov_SantanderMember/ us-gaap_SubsidiarySaleOfStockAxis= us-gaap_LongTermDebtMember          |   |   |   |
| Banco Santander [Member]                                                                              |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| **Class of Stock [Line Items]**                                                                       |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| Issuance of common stock, shares                                                                      |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   | 0us-gaap_StockIssuedDuringPeriodSharesNewIssues/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember | 0us-gaap_StockIssuedDuringPeriodSharesNewIssues/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |
| Proceeds from the issuance of common stock                                                            | $ 1,771,000us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember | $ 0us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember | $ 0us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |                                                                                                                                                               | $ 21,000us-gaap_ProceedsFromIssuanceOfCommonStock/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember |   |   |
| Banco Santander [Member] | Stock Issued for Cash [Member]                                             |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| **Class of Stock [Line Items]**                                                                       |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               |   |   |   |
| Issuance of common stock, shares                                                                      |                                                                                                   |                                                                                           |                                                                                           |                                                                                                                                                               | 84,000us-gaap_StockIssuedDuringPeriodSharesNewIssues/ dei_LegalEntityAxis= us-gaap_ParentCompanyMember/ us-gaap_SubsidiarySaleOfStockAxis= sov_StockIssuedforCashMember |   |   |

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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash inflow from the additional capital contribution to the entity.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of new stock issued during the period.
+ References
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