| **VARIABLE INTEREST ENTITIES (Narrative) (Details) - USD ($)**               | 3 Months Ended  | 8 Months Ended  | 9 Months Ended  |                 |
|------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|
||
| **Variable Interest Entity [Line Items]**                                    |            |            |            |            |   |
| Proceeds from sale of residual interests and retained bonds                  | $ 661,700,000   |            |            |            |   |
| Investments in notes issued by SDART trusts, carrying value                  | 5,600,000       |            |            | $ 5,600,000     |   |
| Derecognized assets                                                          | 1,900,000,000   |            |            | 1,900,000,000   |   |
| Derecognized notes payable and other liabilities                             | 1,200,000,000   |            |            | 1,200,000,000   |   |
| Servicing Fees                                                               | 5,900,000       |            |            |            |   |
| Trusts | VIE, Not Primary Beneficiary                                        |            |            |            |            |   |
| **Variable Interest Entity [Line Items]**                                    |            |            |            |            |   |
| Gross retail installment contracts transferred to consolidated Trusts        | 26,900,000,000  |            |            | 26,900,000,000  | $ 23,200,000,000 |
| Off-balance Securitization Trusts                                            |            |            |            |            |   |
| **Variable Interest Entity [Line Items]**                                    |            |            |            |            |   |
| Proceeds from securitization of retail installment contracts                 | 0               | $ 1,000,000,000 | $ 1,800,000,000 | 768,600,000     |   |
| Gain from securitization of financial assets                                 |            | 39,100,000      | 71,600,000      |            |   |
| Chrysler Capital Securitizations | VIE, Not Primary Beneficiary              |            |            |            |            |   |
| **Variable Interest Entity [Line Items]**                                    |            |            |            |            |   |
| Gross retail installment contracts sold in off-balance sheet securitizations | $ 2,200,000,000 | $ 2,400,000,000 | $ 2,400,000,000 | $ 2,200,000,000 | $ 2,200,000,000 |

| X                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deconsolidation, Notes Issued, Retained Investment
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Derecognized Financial Liabilities, Notes Payable and Other Liabilities
+ References
No definition available.
+ Details**Name:**sov_DerecognizedFinancialLiabilitiesNotesPayableandOtherLiabilities**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Proceeds from Securitization of Retail Installment Contracts
+ References
No definition available.
+ Details**Name:**sov_ProceedsfromSecuritizationofRetailInstallmentContracts**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Proceeds from Sale of Trust Residual Interest
+ References
No definition available.
+ Details**Name:**sov_Sov_ProceedsfromSaleofTrustResidualInterest**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Principal amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=51814546&loc=d3e107314-111719
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction that have been derecognized. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=51814546&loc=d3e107314-111719
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A gain (loss) realized on a transfer, accounted for as a sale, of all or a portion of financial assets in which the transferor surrenders control and receives consideration other than a beneficial interest in the assets transferred. Reflects the amount of sales proceeds in excess of, or deficient from, the sum of the carrying amounts of transferred financial assets plus transaction costs.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of fees from providing shareholder services, including, but not limited to, answering shareholder inquiries and providing shareholders with information about their investments.
+ References
No definition available.
+ Details**Name:**us-gaap_ServicingFees**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of derecognized financial assets from transactions which comprise an initial transfer and an agreement entered into in contemplation of the initial transfer resulting in retention of substantially all of the exposure to the economic return throughout the term of the transaction, at the date of derecognition.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51814546&loc=SL51823488-111719
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| 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_VariableInterestEntityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                  |
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