| **BORROWINGS (Santander Bank) (Details) - USD ($) $ in Thousands**                                                                                                                                                                                                                                                              | 1 Months Ended | 3 Months Ended |           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------|
||
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Total borrowings and other debt obligations                                                                                                                                                                                                                                                                                     | $ 39,679,382   | [1]            |      |   |   | $ 42,774,513 |   |   |
| Loss on debt extinguishment                                                                                                                                                                                                                                                                                                     |           |                |      |   |   | 0 | $ 3,635 |   |
| Santander Bank [Member]                                                                                                                                                                                                                                                                                                         |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Total borrowings and other debt obligations                                                                                                                                                                                                                                                                                     | $ 10,276,555   |                |      |   |   | $ 11,479,907 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 2.64%          |                |      |   |   | 2.37% |   |   |
| Santander Bank [Member] | FHLB Advances, maturing through August 2018 [Member]                                                                                                                                                                                                                                                  |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Repayment of debt                                                                                                                                                                                                                                                                                                               | $ 200,000      |                | $ 600,000 | $ 100,000 |   |   |   |   |
| Loss on debt extinguishment                                                                                                                                                                                                                                                                                                     | 13,700         |                | $ 90,600  | $ 8,300 |   |   |   |   |
| Santander Bank [Member] | 2.00% senior notes, due January 2018 [Member]                                                                                                                                                                                                                                                         |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           | $ 0            |                |      |   |   | $ 744,526 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 0.00%          |                |      |   |   | 2.27% |   |   |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |           |                |      |   |   | 2.00% |   |   |
| Santander Bank [Member] | Senior Notes due January 2018 [Member]                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           | $ 0            |                |      |   |   | $ 249,005 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 0.00%          |                |      |   |   | 1.33% |   |   |
| Basis spread on LIBOR                                                                                                                                                                                                                                                                                                           |           |                |      |   |   | 0.93% |   |   |
| Santander Bank [Member] | 8.750% subordinated debentures, due May 2018 [Member]                                                                                                                                                                                                                                                 |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Subordinated debentures, Balance                                                                                                                                                                                                                                                                                                | $ 497,530      |                |      |   |   | $ 497,686 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 8.92%          |                |      |   |   | 8.91% |   |   |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |           |                |      |   |   | 8.75% |   |   |
| Santander Bank [Member] | FHLB Advances, maturing through August 2018 [Member]                                                                                                                                                                                                                                                  |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| FHLB advances, maturing through August 2018, Balance                                                                                                                                                                                                                                                                            | $ 9,455,000    |                |      |   |   | $ 9,670,000 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 2.06%          |                |      |   |   | 1.82% |   |   |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |           |                |      |   | 3.43% |   |   | 5.39% |
| Santander Bank [Member] | FHLB Advances, maturing through August 2018 [Member] | Restructured FHLB Advances [Member]                                                                                                                                                                                                            |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| FHLB advances, maturing through August 2018, Balance                                                                                                                                                                                                                                                                            |           |                |      |   | $ 1,700,000 |   |   |   |
| Debt maturity, extended term                                                                                                                                                                                                                                                                                                    |           |                |      |   | 2 years |   |   |   |
| Santander Bank [Member] | Subordinated term loan, due February 2019 [Member]                                                                                                                                                                                                                                                    |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Subordinated term loan, Balance                                                                                                                                                                                                                                                                                                 | $ 139,180      |                |      |   |   | $ 133,364 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 6.00%          |                |      |   |   | 6.03% |   |   |
| Santander Bank [Member] | REIT preferred, due May 2020 [Member]                                                                                                                                                                                                                                                                 |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| REIT preferred, due May 2020, Balance                                                                                                                                                                                                                                                                                           | $ 153,417      |                |      |   |   | $ 153,794 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 13.64%         |                |      |   |   | 13.61% |   |   |
| Santander Bank [Member] | Subordinated term loan, due August 2022 [Member]                                                                                                                                                                                                                                                      |           |                |      |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |           |                |      |   |   |   |   |   |
| Subordinated term loan, Balance                                                                                                                                                                                                                                                                                                 | $ 31,428       |                |      |   |   | $ 31,532 |   |   |
| Effective Rate                                                                                                                                                                                                                                                                                                                  | 7.77%          |                |      |   |   | 7.81% |   |   |
|                                                                                                                                                                                                                                                                                                                                 |
| [1]The Company has interests in certain securitization trusts ("Trusts") that are considered variable interest entities ("VIEs") for accounting purposes. The Company consolidates VIEs where it is deemed the primary beneficiary. See Note 7 to these Condensed Consolidated Financial Statements for additional information. |

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Borrowings and other debt obligations.
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Issuance of Stock, Subsidiary
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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.
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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
Gross amount of debt extinguished.
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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
Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.
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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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