| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES - Commitments to Extend Credit (Details) - USD ($) $ in Billions** | Mar. 31, 2024 | Dec. 31, 2023 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Commitments to extend credit                                                                                    |          |          |
| **Other Commitments [Line Items]**                                                                              |          |          |
| Commitments that can be canceled without notice                                                                 | $ 3.3         | $ 3.1         |

| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments, Can Be Canceled Without Notice
+ References
No definition available.
+ Details**Name:**sov_CommitmentsCanBeCanceledWithoutNotice**Namespace Prefix:**sov_**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                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherCommitmentsAxis=us-gaap_CommitmentsToExtendCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |