| **BORROWINGS (Narrative) (Details) - USD ($)**                                                                                                                                                                                                                                                                                  |     |                   | 1 Months Ended    | 3 Months Ended | 9 Months Ended   |                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----|-------------------|-------------------|----------------|------------------|-----------------|
||
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Total borrowings and other debt obligations                                                                                                                                                                                                                                                                                     | [1] |              |              |           | $ 44,604,370,000 |            |   |   | $ 44,604,370,000 |   | $ 49,828,582,000 |
| Loss on debt extinguishment                                                                                                                                                                                                                                                                                                     |     |              |              |           | 10,228,000       |            |   | $ 0 | $ 88,672,000 | $ 0 |   |
| Senior notes, due November 2017                                                                                                                                                                                                                                                                                                 |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Basis spread on LIBOR                                                                                                                                                                                                                                                                                                           |     |              |              |           |             |            |   |   | 1.45% |   |   |
| Senior Notes | 2.70% senior notes, due May 2019                                                                                                                                                                                                                                                                                 |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Aggregate principal amount                                                                                                                                                                                                                                                                                                      |     | $ 1,000,000,000.0 |              |           |             |            |   |   |   |   |   |
| Stated interest rate                                                                                                                                                                                                                                                                                                            |     | 2.70%             |              |           |             |            |   |   |   |   |   |
| Senior Notes | Senior notes, due November 2017                                                                                                                                                                                                                                                                                  |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Aggregate principal amount                                                                                                                                                                                                                                                                                                      |     | $ 600,000,000     |              |           |             |            |   |   |   |   |   |
| FHLB Advances                                                                                                                                                                                                                                                                                                                   |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Debt terminated                                                                                                                                                                                                                                                                                                                 |     |              |              |           | 750,000,000      | $ 1,500,000,000 | $ 1,300,000,000 |   |   |   |   |
| Loss on debt extinguishment                                                                                                                                                                                                                                                                                                     |     |              |              |           | $ 10,200,000     | $ 45,600,000    | $ 32,900,000 |   |   |   |   |
| Santander Bank | Senior Notes | Senior notes with related party, due August 2017                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Aggregate principal amount                                                                                                                                                                                                                                                                                                      |     |              | $ 1,500,000,000.0 |           |             |            |   |   |   |   |   |
| Basis spread on LIBOR                                                                                                                                                                                                                                                                                                           |     |              | 2.18%             |           |             |            |   |   |   |   |   |
| 3-month LIBOR | Senior Notes | Senior notes, due November 2017                                                                                                                                                                                                                                                                  |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Basis spread on LIBOR                                                                                                                                                                                                                                                                                                           |     | 1.45%             |              |           |             |            |   |   |   |   |   |
| Subsequent Event | FHLB Advances                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |     |              |              |           |             |            |   |   |   |   |   |
| Debt terminated                                                                                                                                                                                                                                                                                                                 |     |              |              | $ 300,000,000  |             |            |   |   |   |   |   |
| Loss on debt extinguishment                                                                                                                                                                                                                                                                                                     |     |              |              | $ 25,600,000   |             |            |   |   |   |   |   |
|                                                                                                                                                                                                                                                                                                                                 |
| [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 6 to these Condensed Consolidated Financial Statements for additional information. |

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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.
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Percentage points added to the reference rate to compute the variable rate on the debt instrument.
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Face (par) amount of debt instrument at time of issuance.
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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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Gross amount of debt extinguished.
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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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