| **INVESTMENT SECURITIES (Other Investments) (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** | 3 Months Ended                                                  |                                                                 |                                                                    |
|-------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------|-----------------------------------------------------------------|--------------------------------------------------------------------|
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| **Investments, Debt and Equity Securities [Abstract]**                                                                        |                                                            |                                                            |                                                               |
| Accrued investment income receivable                                                                                          | $ 68.2us-gaap_AccruedInvestmentIncomeReceivable                 | $ 66.9us-gaap_AccruedInvestmentIncomeReceivable                 |                                                               |
| Stock of FHLB of Pittsburgh and the Federal Reserve Board                                                                     | 829.1us-gaap_FederalHomeLoanBankStockAndFederalReserveBankStock | 817.0us-gaap_FederalHomeLoanBankStockAndFederalReserveBankStock |                                                               |
| FHLB Stock, par value (in usd per share)                                                                                      | $ 100sov_FederalHomeLoanBankStockParValue                       |                                                            |                                                               |
| Purchases of FHLB stock                                                                                                       | 149.3us-gaap_PaymentsToAcquireFederalHomeLoanBankStock          |                                                            |                                                               |
| Purchases of FRB stock at par                                                                                                 | 3.7us-gaap_PaymentsToAcquireFederalReserveStock                 |                                                            |                                                               |
| Sale of FHLB stock                                                                                                            | 140.9us-gaap_ProceedsFromSaleOfFederalHomeLoanBankStock         |                                                            |                                                               |
| Low income housing tax credit investment                                                                                      | 24.6sov_LowIncomeHousingTaxCreditInvestment                     |                                                            |                                                               |
| FHLB CD, carrying amount                                                                                                      |                                                            |                                                            | $ 19.9us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits |

| 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
Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 3 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 4 -Article 7
+ Details**Name:**us-gaap_AccruedInvestmentIncomeReceivable**Namespace Prefix:**us-gaap_**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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest-bearing domestic certificate of deposit liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.12) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 9
+ Details**Name:**us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 outflow to acquire Federal Reserve Bank (FRB) stock.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585
+ Details**Name:**us-gaap_PaymentsToAcquireFederalReserveStock**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 |