| **COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Representation and Warranty Liability) (Details) - USD ($) $ in Thousands** | 1 Months Ended | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------|----------------|----------------|----------------|
||
| **Commitments and Contingencies Disclosure [Abstract]**                                                                   |           |           |           |   |   |   |
| Representation and warranty reserve, reduction, settlement                                                                | $ 24,800       | $ 8,000        |           |   |   |   |
| **Representation and Warranty Reserve [Roll Forward]**                                                                    |           |           |           |   |   |   |
| Beginning Balance                                                                                                         |           |           | $ 23,051       | $ 50,515 | $ 24,266 | $ 54,836 |
| Changes in Estimate                                                                                                       |           |           | 849            | (7,772) | 280 | (7,579) |
| Claims                                                                                                                    |           |           | (892)          | (1,881) | (1,538) | (6,395) |
| Ending Balance                                                                                                            | $ 24,266       |           | $ 23,008       | $ 40,862 | $ 23,008 | $ 40,862 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the reserve for the expected losses related to loans sold into the secondary mortgage market that may be required to repurchase, includes both estimate of expected losses and adjustments to previous estimates of expected losses.
+ References
No definition available.
+ Details**Name:**sov_RepresentationAndWarrantyReserve**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount represents the increase in the representation and warranty reserve due to change in estimate of expected mortgage loan repurchase losses for the previously sold loans.
+ References
No definition available.
+ Details**Name:**sov_RepresentationAndWarrantyReserveAdditionProvision**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The charge-offs represent the actual loss incurred upon completion of processing approved repurchase where the Company has agreed to repurchase the loan and/or indemnify the investor.
+ References
No definition available.
+ Details**Name:**sov_RepresentationAndWarrantyReserveReductionChargeOffs**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Representation and Warranty Reserve [Roll Forward]
+ References
No definition available.
+ Details**Name:**sov_RepresentationAndWarrantyReserveRollForward**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Representation and Warranty Reserve, Reduction, Settlement
+ References
No definition available.
+ Details**Name:**sov_RepresentationandWarrantyReserveReductionSettlement**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_CommitmentsAndContingenciesDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |