| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES - Agreements (Details) - SC - USD ($)** | 1 Months Ended |        |        |
|--------------------------------------------------------------------------------------|----------------|--------|--------|
||
| **Other Commitments [Line Items]**                                                   |           |   |   |
| Minimum sales commitment, charged off loan receivables                               | $ 350,000,000  |   |   |
| Threshold for sales subject to market price check (over)                             | $ 275,000,000  |   |   |
| Minimum sales commitment, loans receivable, written off, remaining                   |           | $ 0    | $ 0    |

| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOff**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum Sales Commitment, Loans Receivable, Written Off, Remaining
+ References
No definition available.
+ Details**Name:**sov_MinimumSalesCommitmentLoansReceivableWrittenOffRemaining**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sales Commitment, Sales Subject to Market Price Check, Threshold
+ References
No definition available.
+ Details**Name:**sov_SalesCommitmentSalesSubjecttoMarketPriceCheckThreshold**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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:**dei_LegalEntityAxis=sov_SantanderConsumerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |