| **BORROWINGS (Santander Bank) (Details) - Santander Bank - USD ($) $ in Thousands** | 12 Months Ended |              |
|-------------------------------------------------------------------------------------|-----------------|--------------|
||
| **Debt Instrument [Line Items]**                                                    |            |         |
| Total borrowings and other debt obligations                                         | $ 15,695,144    | $ 10,276,555 |
| Effective Rate                                                                      | 1.85%           | 2.64%        |
| 2.00% senior notes, due January 2018                                                |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| Senior notes, Balance                                                               | $ 746,381       | $ 0          |
| Effective Rate                                                                      | 2.24%           | 0.00%        |
| Stated interest rate                                                                | 2.00%           |         |
| Senior Notes Due January 2018                                                       |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| Senior notes, Balance                                                               | $ 249,415       | $ 0          |
| Effective Rate                                                                      | 1.31%           | 0.00%        |
| Basis spread on LIBOR                                                               | 93.00%          |         |
| 8.750% subordinated debentures, due May 2018                                        |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| Subordinated debentures, Balance                                                    | $ 498,175       | $ 497,530    |
| Effective Rate                                                                      | 8.92%           | 8.92%        |
| Stated interest rate                                                                | 8.75%           |         |
| FHLB advances, maturing through July 2019                                           |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| FHLB advances                                                                       | $ 13,885,000    | $ 9,455,000  |
| Effective Rate                                                                      | 1.40%           | 2.06%        |
| FHLB advances, convertible from fixed to floating rate                              |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| FHLB advances                                                                       | $ 650,000       | $ 650,000    |
| FHLB advances, recurring callable period                                            | 3 months        |         |
| Subordinated term loan, due February 2019                                           |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| Subordinated term loan, Balance                                                     | $ 130,899       | $ 139,180    |
| Effective Rate                                                                      | 6.15%           | 6.00%        |
| REIT preferred, due May 2020                                                        |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| REIT preferred, due May 2020, Balance                                               | $ 154,930       | $ 153,417    |
| Effective Rate                                                                      | 13.66%          | 13.64%       |
| Subordinated term loan, due August 2022                                             |            |         |
| **Debt Instrument [Line Items]**                                                    |            |         |
| Subordinated term loan, Balance                                                     | $ 30,344        | $ 31,428     |
| Effective Rate                                                                      | 7.89%           | 7.77%        |

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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Borrowings and other debt obligations.
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Issuance of Stock, Subsidiary
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Long-term Federal Home Loan Bank Advances, Callable Period
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Percentage points added to the reference rate to compute the variable rate on the debt instrument.
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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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| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
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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
Including the current and noncurrent portions, aggregate carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer).
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| - Definition
Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.
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| - Definition
Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.
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