| **FAIR VALUE - Fair Value Option for Financial Assets and Liabilities (Details) - USD ($) $ in Thousands** | Sep. 30, 2022 | Dec. 31, 2021 |
|------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value**                                                                                             |          |          |
| Nonaccrual loans                                                                                           | $ 152         | $ 435         |
| **Aggregate UPB**                                                                                          |          |          |
| Nonaccrual loans                                                                                           | 161           | 457           |
| **Difference**                                                                                             |          |          |
| Nonaccrual loans                                                                                           | (9)           | (22)          |
| LHFS                                                                                                       |          |          |
| **Fair Value**                                                                                             |          |          |
| LHFS                                                                                                       | 0             | 166,811       |
| **Aggregate UPB**                                                                                          |          |          |
| LHFS                                                                                                       | 0             | 162,525       |
| **Difference**                                                                                             |          |          |
| LHFS                                                                                                       | 0             | 4,286         |
| RICs HFI                                                                                                   |          |          |
| **Fair Value**                                                                                             |          |          |
| RICs HFI                                                                                                   | 23,283        | 33,529        |
| **Aggregate UPB**                                                                                          |          |          |
| RICs HFI                                                                                                   | 23,691        | 33,737        |
| **Difference**                                                                                             |          |          |
| RICs HFI                                                                                                   | $ (408)       | $ (208)       |

| X                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Option, Aggregate Unpaid Principal Balance [Abstract]
+ References
No definition available.
+ Details**Name:**sov_FairValueOptionAggregateUnpaidPrincipalBalanceAbstract**Namespace Prefix:**sov_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Option, Loans Held as Assets, Aggregate Amount
+ References
No definition available.
+ Details**Name:**sov_FairValueOptionLoansHeldasAssetsAggregateAmount**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Option, Loans Held as Assets, Aggregate Amount in Nonaccrual Status, Aggregate Unpaid Principal Balance
+ References
No definition available.
+ Details**Name:**sov_FairValueOptionLoansHeldasAssetsAggregateAmountinNonaccrualStatusAggregateUnpaidPrincipalBalance**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Option, Loans Held as Assets, Aggregate Unpaid Principal Balance
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Option, Loans Held for Sale, Aggregate Amount
+ References
No definition available.
+ Details**Name:**sov_FairValueOptionLoansHeldforSaleAggregateAmount**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unpaid principal balance of loans held-for-sale.
+ References
No definition available.
+ Details**Name:**sov_LoansHeldForSaleUnpaidPrincipalBalance**Namespace Prefix:**sov_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of loans and long-term receivables (other than securities categorized as trading, available-for-sale or held-to-maturity) that have contractual principal amounts and for which the fair value option has been elected.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (d)(1) -URI https://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612
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| X                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FairValueOptionLoansHeldAsAssetsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of loans held as assets that are in nonaccrual status for which the fair value option has been elected, if the entity's policy is to recognize interest income separately from other changes in fair value.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(2) -URI https://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are in nonaccrual status.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(3) -URI https://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due, in nonaccrual status, or both.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(3) -URI https://asc.fasb.org/extlink&oid=123596393&loc=d3e14064-108612
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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