| **INVESTMENT SECURITIES (Available-for-sale Securities) (Details) - USD ($) $ in Thousands** | 3 Months Ended | 6 Months Ended |              |
|----------------------------------------------------------------------------------------------|----------------|----------------|--------------|
||
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | $ 19,280,209   |           | $ 19,280,209 |   | $ 21,006,066 |
| Gross Unrealized Gains                                                                       | 189,075        |           | 189,075      |   | 63,973 |
| Gross Unrealized Loss                                                                        | (50,671)       |           | (50,671)     |   | (218,544) |
| Fair Value                                                                                   | 19,418,613     |           | 19,418,613   |   | 20,851,495 |
| Proceeds from sales of available-for-sale investment securities                              | 3,319,454      | $ 1,244,093    | 6,755,298    | $ 2,254,608 |   |
| Accrued investment income receivable                                                         | 40,200         |           | 40,200       |   | 65,100 |
| U.S. Treasury securities                                                                     |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 2,004,639      |           | 2,004,639    |   | 3,192,411 |
| Gross Unrealized Gains                                                                       | 114            |           | 114          |   | 1,658 |
| Gross Unrealized Loss                                                                        | 0              |           | 0            |   | (5,681) |
| Fair Value                                                                                   | 2,004,753      |           | 2,004,753    |   | 3,188,388 |
| Corporate debt securities                                                                    |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               |           |           |         |   | 1,476,801 |
| Gross Unrealized Gains                                                                       |           |           |         |   | 10,021 |
| Gross Unrealized Loss                                                                        |           |           |         |   | (11,248) |
| Fair Value                                                                                   |           |           |         |   | 1,475,574 |
| Asset-backed securities (ABS)                                                                |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 1,931,093      |           | 1,931,093    |   | 1,763,178 |
| Gross Unrealized Gains                                                                       | 10,380         |           | 10,380       |   | 7,826 |
| Gross Unrealized Loss                                                                        | (1,630)        |           | (1,630)      |   | (1,494) |
| Fair Value                                                                                   | 1,939,843      |           | 1,939,843    |   | 1,769,510 |
| Equity securities                                                                            |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 11,030         |           | 11,030       |   | 10,894 |
| Gross Unrealized Gains                                                                       | 7              |           | 7            |   | 1 |
| Gross Unrealized Loss                                                                        | (238)          |           | (238)        |   | (408) |
| Fair Value                                                                                   | 10,799         |           | 10,799       |   | 10,487 |
| State and municipal securities                                                               |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 33             |           | 33           |   | 748,696 |
| Gross Unrealized Gains                                                                       | 0              |           | 0            |   | 19,616 |
| Gross Unrealized Loss                                                                        | 0              |           | 0            |   | (432) |
| Fair Value                                                                                   | 33             |           | 33           |   | 767,880 |
| U.S. government agencies - Residential                                                       |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 6,073,061      |           | 6,073,061    |   | 4,033,041 |
| Gross Unrealized Gains                                                                       | 67,550         |           | 67,550       |   | 10,225 |
| Gross Unrealized Loss                                                                        | (9,880)        |           | (9,880)      |   | (36,513) |
| Fair Value                                                                                   | 6,130,731      |           | 6,130,731    |   | 4,006,753 |
| U.S. government agencies - Commercial                                                        |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 995,610        |           | 995,610      |   | 1,006,161 |
| Gross Unrealized Gains                                                                       | 20,841         |           | 20,841       |   | 3,347 |
| Gross Unrealized Loss                                                                        | (1,295)        |           | (1,295)      |   | (7,153) |
| Fair Value                                                                                   | 1,015,156      |           | 1,015,156    |   | 1,002,355 |
| FHLMC and FNMA - Residential debt securities                                                 |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 8,240,765      |           | 8,240,765    |   | 8,636,745 |
| Gross Unrealized Gains                                                                       | 89,550         |           | 89,550       |   | 10,556 |
| Gross Unrealized Loss                                                                        | (37,620)       |           | (37,620)     |   | (153,128) |
| Fair Value                                                                                   | 8,292,695      |           | 8,292,695    |   | 8,494,173 |
| FHLMC and FNMA - Commercial debt securities                                                  |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 23,948         |           | 23,948       |   | 138,094 |
| Gross Unrealized Gains                                                                       | 633            |           | 633          |   | 723 |
| Gross Unrealized Loss                                                                        | (8)            |           | (8)          |   | (2,487) |
| Fair Value                                                                                   | 24,573         |           | 24,573       |   | 136,330 |
| Non-agency securities                                                                        |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Amortized Cost                                                                               | 30             |           | 30           |   | 45 |
| Gross Unrealized Gains                                                                       | 0              |           | 0            |   | 0 |
| Gross Unrealized Loss                                                                        | 0              |           | 0            |   | 0 |
| Fair Value                                                                                   | $ 30           |           | $ 30         |   | $ 45 |
| FHLMC | FHLMC and FNMA - Residential debt securities                                         |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| MBS issued by FHLMC                                                                          |           | 243,900        |         | $ 243,900 |   |
| FHLMC | Residential mortgages                                                                |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Proceeds from sales of available-for-sale investment securities                              |           | 234,500        |         |   |   |
| FHLMC | Mortgage banking income                                                              |           |           |         |   |   |
| **Schedule of Available-for-sale Securities [Line Items]**                                   |           |           |         |   |   |
| Gain on sale of MBS                                                                          |           | $ 9,800        |         |   |   |

| 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 FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(4)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 3: 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(10)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
+ Details**Name:**us-gaap_AccruedInvestmentIncomeReceivable**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882
+ Details**Name:**us-gaap_GainLossOnSalesOfMortgageBackedSecuritiesMBS**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Mortgage-Backed Securities -URI http://asc.fasb.org/extlink&oid=6517959
+ Details**Name:**us-gaap_MortgageBackedSecuritiesAvailableForSaleFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26853-111562Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585
+ Details**Name:**us-gaap_ProceedsFromSaleOfAvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasurySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MunicipalBondsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_CommercialmortgagebackedsecuritiesissuedbyotherGovernmentagenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=sov_CommercialMortgagebackedSecuritiesIssuedbyUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_FederalHomeLoanMortgageCorporationFhlmcInsuredLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IncomeStatementLocationAxis=sov_MortgageBankingIncomeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |