| **BORROWINGS - Santander Bank (Details) - SBNA - USD ($)** | 6 Months Ended  |               |
|------------------------------------------------------------|-----------------|---------------|
||
| **Debt Instrument [Line Items]**                           |            |          |
| Total SBNA borrowings and other debt obligations           | $ 3,469,015,000 | $ 543,749,000 |
| Effective Rate                                             | 2.52%           | 1.79%         |
| FHLB advances, maturing through August 2022                |            |          |
| **Debt Instrument [Line Items]**                           |            |          |
| FHLB advances, Balance                                | $ 2,750,000,000 | $ 250,000,000 |
| Effective Rate                                             | 1.83%           | 0.93%         |
| Credit linked notes due December 2031                      |            |          |
| **Debt Instrument [Line Items]**                           |            |          |
| Credit linked notes, due balance                           | $ 224,126,000   | $ 293,749,000 |
| Effective Rate                                             | 2.67%           | 2.53%         |
| Debt issued                                                | $ 521,000,000   | $ 298,000,000 |
| Reference pool                                             |            | 2,000,000,000 |
| Contractual residual amount                                |            | 36,000,000    |
| Redemption option, UPB threshold                           | 10.00%          |          |
| Credit linked notes due May 3032                           |            |          |
| **Debt Instrument [Line Items]**                           |            |          |
| Credit linked notes, due balance                           | $ 494,889,000   | $ 0           |
| Effective Rate                                             | 6.30%           | 0.00%         |
| Reference pool                                             | $ 3,500,000,000 |          |
| Contractual residual amount                                | 63,000,000      |          |
| FHLB advances                                              |            |          |
| **Debt Instrument [Line Items]**                           |            |          |
| Letters of credit                                          | $ 12,000,000    |          |

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Borrowings and other debt obligations.
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Debt Instrument, Redemption Option, Unpaid Principal Balance Threshold
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Debt Instrument, Reference Pool
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.
+ References
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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.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Including both current and noncurrent portions, Federal Home Loan Bank borrowings initially due beyond one year or beyond the normal operating cycle if longer.
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| - Definition
The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.
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