| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Non-accrual Loans) (Details) - USD ($) $ in Thousands** | Sep. 30, 2015 | Dec. 31, 2014 |
|--------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 1,470,616   | $ 1,596,509   |
| Foreclosed and other repossessed assets                                                          | 154,433       | 137,201       |
| Nonperforming                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Other real estate owned (OREO)                                                                   | 40,692        | 65,051        |
| Repossessed vehicles                                                                             | 154,056       | 136,136       |
| Foreclosed and other repossessed assets                                                          | 377           | 11,375        |
| Total OREO and other repossessed assets                                                          | 195,125       | 212,562       |
| Total non-performing assets                                                                      | 1,665,741     | 1,809,071     |
| Commercial                                                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 180,267       | 236,213       |
| Consumer                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 1,290,349     | 1,360,296     |
| Corporate banking                                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 60,450        | 90,579        |
| Middle market commercial real estate                                                             |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 38,605        | 71,398        |
| Santander real estate capital                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 3,505         | 5,803         |
| Commercial and industrial                                                                        |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 66,023        | 54,658        |
| Multi-family                                                                                     |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 7,319         | 9,639         |
| Other commercial                                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 4,365         | 4,136         |
| Residential mortgages                                                                            |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 184,828       | 231,316       |
| Home equity loans and lines of credit                                                            |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 126,628       | 142,026       |
| Retail installment contracts and auto loans                                                      |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 936,191       | 960,293       |
| Personal unsecured loans                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | 12,683        | 14,007        |
| Other consumer                                                                                   |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                             |          |          |
| Non-accrual loans                                                                                | $ 30,019      | $ 12,654      |

| X                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Repossessed Vehicles
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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