| **DERIVATIVES (Other Derivatives) (Details) - USD ($) $ in Thousands**                                  | 1 Months Ended |              |              |
|---------------------------------------------------------------------------------------------------------|----------------|--------------|--------------|
||
| Not designated as hedging instrument                                                                    |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | $ 48,227,817 | $ 38,615,451 |
| Asset derivatives Fair value                                                                            |           | 354,293      | 376,608      |
| Liability derivatives Fair value                                                                        |           | 319,296      | 352,680      |
| Not designated as hedging instrument | Other                                                            |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 899,394      | 646,321      |
| Asset derivatives Fair value                                                                            |           | 11,146       | 6,543        |
| Liability derivatives Fair value                                                                        |           | 14,553       | 9,914        |
| Not designated as hedging instrument | Foreign exchange contracts                                       |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 2,513,305    | 1,152,125    |
| Asset derivatives Fair value                                                                            |           | 30,262       | 20,033       |
| Liability derivatives Fair value                                                                        |           | 30,144       | 17,390       |
| Not designated as hedging instrument | Interest rate swap agreements                                    |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 2,399,000    | 3,231,000    |
| Asset derivatives Fair value                                                                            |           | 1,176        | 535          |
| Liability derivatives Fair value                                                                        |           | 2,481        | 12,743       |
| Not designated as hedging instrument | Interest rate cap agreements                                     |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 10,013,912   | 7,541,385    |
| Asset derivatives Fair value                                                                            |           | 32,950       | 49,762       |
| Liability derivatives Fair value                                                                        |           | 0            | 0            |
| Not designated as hedging instrument | Options for interest rate cap agreements                         |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 10,013,912   | 7,541,385    |
| Asset derivatives Fair value                                                                            |           | 0            | 0            |
| Liability derivatives Fair value                                                                        |           | 32,977       | 49,806       |
| Not designated as hedging instrument | Mortgage banking derivatives                                     |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 1,019,448    | 960,770      |
| Asset derivatives Fair value                                                                            |           | 3,761        | 10,495       |
| Liability derivatives Fair value                                                                        |           | 3,502        | 9,872        |
| Not designated as hedging instrument | Mortgage banking derivatives | Forward commitments to sell loans |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 396,518      | 328,757      |
| Asset derivatives Fair value                                                                            |           | 542          | 0            |
| Liability derivatives Fair value                                                                        |           | 0            | 2,424        |
| Not designated as hedging instrument | Mortgage banking derivatives | Interest rate lock commitments    |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 187,930      | 163,013      |
| Asset derivatives Fair value                                                                            |           | 2,540        | 3,063        |
| Liability derivatives Fair value                                                                        |           | 0            | 0            |
| Not designated as hedging instrument | Mortgage banking derivatives | Mortgage servicing                |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 435,000      | 469,000      |
| Asset derivatives Fair value                                                                            |           | 679          | 7,432        |
| Liability derivatives Fair value                                                                        |           | 3,502        | 7,448        |
| Not designated as hedging instrument | Customer related derivatives                                     |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 21,368,846   | 17,542,465   |
| Asset derivatives Fair value                                                                            |           | 274,998      | 289,240      |
| Liability derivatives Fair value                                                                        |           | 235,639      | 252,955      |
| Not designated as hedging instrument | Customer related derivatives | Swaps receive fixed               |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 9,060,134    | 7,927,522    |
| Asset derivatives Fair value                                                                            |           | 205,397      | 213,415      |
| Liability derivatives Fair value                                                                        |           | 6,023        | 4,343        |
| Not designated as hedging instrument | Customer related derivatives | Swaps pay fixed                   |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 9,273,627    | 7,944,247    |
| Asset derivatives Fair value                                                                            |           | 16,183       | 13,361       |
| Liability derivatives Fair value                                                                        |           | 177,114      | 186,732      |
| Not designated as hedging instrument | Customer related derivatives | Other                             |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | 3,035,085    | 1,670,696    |
| Asset derivatives Fair value                                                                            |           | 53,418       | 62,464       |
| Liability derivatives Fair value                                                                        |           | 52,502       | 61,880       |
| SC                                                                                                      |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Notional                                                                                                |           | $ 13,700,000 | $ 16,300,000 |
| SC | Third party                                                                                        |           |         |         |
| **Derivative [Line Items]**                                                                             |           |         |         |
| Proceeds from issuance of debt pledged with certain bonds                                               | $ 250,600      |         |         |
| Three-Month LIBOR [Member] | SC | Third party                                                           |           |         |         |
| **Derivative Instruments Not Designated as Hedging Instruments [Abstract]**                             |           |         |         |
| Basis spread on LIBOR                                                                                   | 75.00%         |         |         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B
+ Details**Name:**invest_DerivativeNotionalAmount**Namespace Prefix:**invest_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage points added to the reference rate to compute the variable rate on the debt instrument.
+ References
No definition available.
+ Details**Name:**us-gaap_DebtInstrumentBasisSpreadOnVariableRate1**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624163-113959
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| 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_DerivativeLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.
+ References
No definition available.
+ Details**Name:**us-gaap_ProceedsFromIssuanceOfDebt**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                        |
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| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
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| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                |
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| X                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=sov_SCMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
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| - Details**Name:**us-gaap_LineOfCreditFacilityAxis=sov_ThirdPartyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                          |
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