| **INVESTMENT SECURITIES (AFS Debt Securities) (Details) - USD ($) $ in Thousands** | Sep. 30, 2019 | Dec. 31, 2018 |
|------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | $ 13,712,724  | $ 11,934,313  |
| Gross Unrealized Gains                                                             | 60,029        | 6,851         |
| Gross Unrealized Loss                                                              | (41,529)      | (308,177)     |
| Fair Value                                                                         | 13,731,224    | 11,632,987    |
| U.S. Treasury securities                                                           |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 5,061,127     | 1,815,914     |
| Gross Unrealized Gains                                                             | 4,361         | 560           |
| Gross Unrealized Loss                                                              | (1,942)       | (11,729)      |
| Fair Value                                                                         | 5,063,546     | 1,804,745     |
| Corporate debt securities                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 136,530       | 160,164       |
| Gross Unrealized Gains                                                             | 51            | 12            |
| Gross Unrealized Loss                                                              | (42)          | (62)          |
| Fair Value                                                                         | 136,539       | 160,114       |
| Asset-backed securities (“ABS”)                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 154,501       | 435,464       |
| Gross Unrealized Gains                                                             | 1,537         | 3,517         |
| Gross Unrealized Loss                                                              | (1,500)       | (2,144)       |
| Fair Value                                                                         | 154,538       | 436,837       |
| State and municipal securities                                                     |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 11            | 16            |
| Gross Unrealized Gains                                                             | 0             | 0             |
| Gross Unrealized Loss                                                              | 0             | 0             |
| Fair Value                                                                         | 11            | 16            |
| GNMA - Residential                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 2,858,083     | 2,829,075     |
| Gross Unrealized Gains                                                             | 12,806        | 861           |
| Gross Unrealized Loss                                                              | (9,770)       | (85,675)      |
| Fair Value                                                                         | 2,861,119     | 2,744,261     |
| GNMA - Commercial                                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 725,851       | 954,651       |
| Gross Unrealized Gains                                                             | 12,228        | 1,250         |
| Gross Unrealized Loss                                                              | (64)          | (19,515)      |
| Fair Value                                                                         | 738,015       | 936,386       |
| FHLMC and FNMA - Residential                                                       |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 4,706,518     | 5,687,221     |
| Gross Unrealized Gains                                                             | 25,327        | 267           |
| Gross Unrealized Loss                                                              | (28,206)      | (188,515)     |
| Fair Value                                                                         | 4,703,639     | 5,498,973     |
| FHLMC and FNMA - Commercial                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                               |          |          |
| Amortized Cost                                                                     | 70,103        | 51,808        |
| Gross Unrealized Gains                                                             | 3,719         | 384           |
| Gross Unrealized Loss                                                              | (5)           | (537)         |
| Fair Value                                                                         | $ 73,817      | $ 51,655      |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454
+ Details**Name:**us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesDebtSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=sov_ResidentialMortgageBackedSecuritiesIssuedByOtherGovermentAgenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=sov_CommercialmortgagebackedsecuritiesissuedbyotherGovernmentagenciesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=sov_CommercialMortgagebackedSecuritiesIssuedbyUSGovernmentSponsoredEnterprisesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |