| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Letters of Credit) (Details) - USD ($) $ in Thousands** | 6 Months Ended          |           |             |           |           |           |
|-------------------------------------------------------------------------------------------------------|-------------------------|-----------|-------------|-----------|-----------|-----------|
||
| **Commitments [Line Items]**                                                                          |                    |      |        |      |      |      |
| Reserve for unfunded lending commitments                                                              | $ 137,641               | $ 127,641 | $ 132,641   | $ 170,274 | $ 180,000 | $ 220,000 |
| Lines of credit outstanding                                                                           | $ 97,900                |      | 58,800      |      |      |      |
| Letters of credit [Member]                                                                            |                    |      |        |      |      |      |
| **Commitments [Line Items]**                                                                          |                    |      |        |      |      |      |
| Commitments, weighted average term                                                                    | 1 year 3 months 27 days |      |        |      |      |      |
| Reserve for unfunded lending commitments                                                              | $ 39,800                |      | 73,900      |      |      |      |
| One year or less                                                                                      | 1,203,983               |      | 1,250,124   |      |      |      |
| Over 1 year to 3 years                                                                                | 270,046                 |      | 285,108     |      |      |      |
| Over 3 years to 5 years                                                                               | 307,027                 |      | 248,209     |      |      |      |
| Over 5 years                                                                                          | 21,398                  |      | 6,225       |      |      |      |
| Total                                                                                                 | $ 1,802,454             |      | $ 1,789,666 |      |      |      |

| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments
+ References
No definition available.
+ Details**Name:**sov_CommitmentsLineItems**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average term of commitment
+ References
No definition available.
+ Details**Name:**sov_CommitmentsWeightedAverageTerm**Namespace Prefix:**sov_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the amount committed expiring in next twelve months under commitments.
+ References
No definition available.
+ Details**Name:**sov_FairValueDisclosureOffBalanceSheetRisksFaceAmountLiabilityLessThanOneYear**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability, Over Five Years
+ References
No definition available.
+ Details**Name:**sov_FairValueDisclosureOffBalanceSheetRisksFaceAmountLiabilityOverFiveYears**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents amount committed expiring in the next two to three years under commitments.
+ References
No definition available.
+ Details**Name:**sov_FairValueDisclosureOffBalanceSheetRisksFaceAmountLiabilityOverOneYearToThreeYears**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability, Over Three Years to Five Years
+ References
No definition available.
+ Details**Name:**sov_FairValueDisclosureOffBalanceSheetRisksFaceAmountLiabilityOverThreeYearsToFiveYears**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reserve for unfunded loan commitments and letters of credit.
+ References
No definition available.
+ Details**Name:**sov_ReserveForUnfundedCommitments**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.
+ References
No definition available.
+ Details**Name:**us-gaap_LettersOfCreditOutstandingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5
+ Details**Name:**us-gaap_LineOfCredit**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_CommitmentsTypeAxis=us-gaap_LetterOfCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |