| **VARIABLE INTEREST ENTITIES (Assets and Liabilities of VIEs) (Details) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------|
| **Variable Interest Entity and Securitizations [Abstract]**                                                               |                                                                                                          |
| Restricted cash                                                                                                           | $ 1,852,825sov_VariableInterestEntityConsolidatedCarryingAmountRestrictedCash                                 |
| Loans held for investment                                                                                                 | 20,555,636sov_VariableInterestEntityConsolidatedCarryingAmountRetailInstallmentContractsNet                   |
| Leased vehicles, net                                                                                                      | 5,042,419sov_VariableInterestEntityConsolidatedCarryingAmountPropertySubjecttoorAvailableforOperatingLeaseNet |
| Various other assets                                                                                                      | 2,393,301sov_VariableInterestEntityConsolidatedCarryingAmountOtherAssets                                      |
| Notes payable                                                                                                             | 29,755,859sov_VariableInterestEntityConsolidatedCarryingAmountNotesPayable                                    |
| Various other liabilities                                                                                                 | $ 2,086sov_VariableInterestEntityConsolidatedCarryingAmountOtherLiabilities                                   |

| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**sov_VariableInterestEntityandSecuritizationsAbstract**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Notes Payable
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountNotesPayable**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Other Assets
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountOtherAssets**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Other Liabilities
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountOtherLiabilities**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Property Subject to or Available for Operating Lease, Net
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountPropertySubjecttoorAvailableforOperatingLeaseNet**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Restricted Cash
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountRestrictedCash**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Variable Interest Entity, Consolidated, Carrying Amount, Retail Installment Contracts, Net
+ References
No definition available.
+ Details**Name:**sov_VariableInterestEntityConsolidatedCarryingAmountRetailInstallmentContractsNet**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |