| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Non-accrual Loans) (Details) - USD ($) $ in Thousands** | Jun. 30, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 1,783,830   | $ 1,655,099   |
| Foreclosed and other repossessed assets                                                          | 186,202       | 172,749       |
| Nonperforming                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Other real estate owned (OREO)                                                                   | 37,432        | 38,959        |
| Repossessed vehicles                                                                             | 183,713       | 172,375       |
| Foreclosed and other repossessed assets                                                          | 2,489         | 374           |
| Total OREO and other repossessed assets                                                          | 223,634       | 211,708       |
| Total non-performing assets                                                                      | 2,007,464     | 1,866,807     |
| Commercial                                                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 412,313       | 211,067       |
| Commercial | Corporate banking                                                                   |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 68,537        | 71,979        |
| Commercial | Middle market commercial real estate                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 74,799        | 37,745        |
| Commercial | Santander real estate capital                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 3,210         | 3,454         |
| Commercial | Commercial and industrial                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 248,688       | 85,745        |
| Commercial | Multifamily                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 6,845         | 9,162         |
| Commercial | Other commercial                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 10,234        | 2,982         |
| Consumer                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 1,371,517     | 1,444,032     |
| Consumer | Residential mortgages                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 155,738       | 173,780       |
| Consumer | Home equity loans and lines of credit                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 119,135       | 127,171       |
| Consumer | Retail installment contracts and auto loans - originated                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 766,982       | 701,785       |
| Consumer | Retail installment contracts and auto loans - purchased                               |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 315,414       | 417,276       |
| Consumer | Personal unsecured loans                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 207           | 895           |
| Consumer | Other consumer                                                                        |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 14,041      | $ 23,125      |

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Repossessed Vehicles
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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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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.
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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.
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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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