| **RELATED PARTY TRANSACTIONS - Letters of Credit (Details) $ in Millions** | 12 Months Ended |
|----------------------------------------------------------------------------|-----------------|
||
| BSI | Affiliates                                                           |            |   |
| **Related Party Transaction [Line Items]**                                 |            |   |
| Letters of credit                                                          | $ 292.4         | $ 224.6 |
| Letters of credit | Agreement Between Santander and Bank                   |            |   |
| **Related Party Transaction [Line Items]**                                 |            |   |
| Average unfunded balance outstanding                                       | $ 162.4         | $ 146.9 |
| Letters of credit | Santander New York Branch on Behalf of the Bank        |            |   |
| **Related Party Transaction [Line Items]**                                 |            |   |
| Number of letters of credit issued | letterOfCredit                        | 2               |   |

| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line of Credit Facility, Average Unfunded Balance Outstanding
+ References
No definition available.
+ Details**Name:**sov_LineofCreditFacilityAverageUnfundedBalanceOutstanding**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Letters of Credit Issued
+ References
No definition available.
+ Details**Name:**sov_NumberOfLettersOfCreditIssued**Namespace Prefix:**sov_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| 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
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_RelatedPartyTransactionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_BSIMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionAxis=sov_AgreementBetweenSantanderandBankMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RelatedPartyTransactionAxis=sov_SantanderNewYorkBranchOnBehalfOfTheBankMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |