| **INVESTMENT SECURITIES (Other Investments) (Details) - USD ($) $ / shares in Units, $ in Millions** | 3 Months Ended | 6 Months Ended |         |
|------------------------------------------------------------------------------------------------------|----------------|----------------|---------|
||
| **Investments, Debt and Equity Securities [Abstract]**                                               |           |           |    |
| Stock of FHLB of Pittsburgh and the Federal Reserve Bank                                             | $ 801.8        | $ 801.8        | $ 997.9 |
| FHLB Stock, par value (in usd per share)                                                             | $ 100          | $ 100          |    |
| Purchases of FHLB stock                                                                              | $ 19.4         | $ 94.4         |    |
| FHLB stock redeemed                                                                                  | 170.3          | 290.4          |    |
| Low income housing tax credit investment                                                             | $ 34.5         | $ 34.5         | $ 26.4  |

| X                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal Home Loan Bank Stock, par value
+ References
No definition available.
+ Details**Name:**sov_FederalHomeLoanBankStockParValue**Namespace Prefix:**sov_**Data Type:**num:perShareItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Low Income Housing Tax Credit Investment
+ References
No definition available.
+ Details**Name:**sov_LowIncomeHousingTaxCreditInvestment**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of Federal Home Loan Bank (FHLB) and Federal Reserve Bank stock.
+ References
No definition available.
+ Details**Name:**us-gaap_FederalHomeLoanBankStockAndFederalReserveBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash outflow to acquire Federal Home Loan Bank (FHLB) stock.
+ References
No definition available.
+ Details**Name:**us-gaap_PaymentsToAcquireFederalHomeLoanBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash inflow from sale of Federal Home Loan Bank (FHLB) stock.
+ References
No definition available.
+ Details**Name:**us-gaap_ProceedsFromSaleOfFederalHomeLoanBankStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |