| **COMMITMENTS, CONTINGENCIES AND GUARANTEES (Loans Sold with Recourse and Commitments to Sell Loans) (Details) - USD ($) $ in Millions** | 3 Months Ended |         |
|------------------------------------------------------------------------------------------------------------------------------------------|----------------|---------|
||
| Recourse exposure on sold loans                                                                                                          |           |    |
| **Other Commitments [Line Items]**                                                                                                       |           |    |
| Retained credit risk, multifamily servicing                                                                                              | $ 6.4          | $ 6.8   |
| Recourse exposure on sold loans | FNMA | Multifamily loans                                                                               |           |    |
| **Other Commitments [Line Items]**                                                                                                       |           |    |
| Loans sold with recourse, portion of first credit loss position retained, percent                                                        | 100.00%        |    |
| Value of underlying collateral                                                                                                           | $ 34.4         | 34.4    |
| Commitments to sell loans                                                                                                                |           |    |
| **Other Commitments [Line Items]**                                                                                                       |           |    |
| Forward contracts maturity period (less than)                                                                                            | 1 year         |    |
| Multifamily loans | Recourse exposure on sold loans | FNMA                                                                               |           |    |
| **Other Commitments [Line Items]**                                                                                                       |           |    |
| Loans sold with recourse, unpaid principal balance                                                                                       | $ 543.7        | $ 552.1 |

| X                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Portion (in percent) of first credit loss position on loans sold with recourse retained.
+ References
No definition available.
+ Details**Name:**sov_LoansSoldWithRecoursePortionOfFirstCreditLossPositionRetainedPercent**Namespace Prefix:**sov_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of loans sold with recourse at period end.
+ References
No definition available.
+ Details**Name:**sov_LoansSoldWithRecourseUnpaidPrincipalBalance**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retained Credit Risk, Multifamily Servicing
+ References
No definition available.
+ Details**Name:**sov_RetainedCreditRiskMultifamilyServicing**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
High end period remaining until maturity for the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeHigherRemainingMaturityRange1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248
+ Details**Name:**us-gaap_GuaranteeObligationsMaximumExposure**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_OtherCommitmentsLineItems**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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| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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