| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Non-accrual Loans) (Details) - USD ($) $ in Thousands** | Mar. 31, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 1,522,099   | $ 1,640,154   |
| Foreclosed and other repossessed assets                                                          | 190,565       | 204,494       |
| Nonperforming                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Other real estate owned (OREO)                                                                   | 36,981        | 38,959        |
| Repossessed vehicles                                                                             | 188,744       | 172,375       |
| Foreclosed and other repossessed assets                                                          | 1,821         | 374           |
| Total OREO and other repossessed assets                                                          | 227,546       | 211,708       |
| Total non-performing assets                                                                      | 1,749,645     | 1,851,862     |
| Commercial                                                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 415,824       | 211,250       |
| Commercial | Corporate banking                                                                   |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 86,397        | 71,979        |
| Commercial | Middle market commercial real estate                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 69,249        | 37,745        |
| Commercial | Santander real estate capital                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 2,754         | 3,454         |
| Commercial | Commercial and industrial                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 249,610       | 85,928        |
| Commercial | Multifamily                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 4,855         | 9,162         |
| Commercial | Other commercial                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 2,959         | 2,982         |
| Consumer                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 1,106,275     | 1,428,904     |
| Consumer | Residential mortgages                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 168,212       | 173,780       |
| Consumer | Home equity loans and lines of credit                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 123,883       | 127,171       |
| Consumer | Retail installment contracts and auto loans                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 798,401       | 1,104,986     |
| Consumer | Personal unsecured loans                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 400           | 895           |
| Consumer | Other consumer                                                                        |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 15,379      | $ 22,072      |

| X                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Repossessed Vehicles
+ References
No definition available.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
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| - Definition
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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No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.
+ References
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| - Definition
Carrying amount as of the balance sheet date of assets other than real property that were received in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.
+ References
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