| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Lending Asset Quality Indicators) (Details) - USD ($) $ in Thousands** | Mar. 31, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | $ 41,719,535  | $ 40,709,731  |
| Pass                                                                                                            |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 39,377,093    | 38,670,976    |
| Special Mention                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 1,001,545     | 1,043,286     |
| Substandard                                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 1,240,615     | 926,537       |
| Doubtful                                                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 100,282       | 68,932        |
| Commercial and industrial loans                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 20,334,247    | 19,871,845    |
| Commercial and industrial loans | Pass                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 18,881,975    | 18,878,763    |
| Commercial and industrial loans | Special Mention                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 587,511       | 492,127       |
| Commercial and industrial loans | Substandard                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 833,212       | 467,983       |
| Commercial and industrial loans | Doubtful                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 31,549        | 32,972        |
| Multifamily                                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 9,330,482     | 9,438,463     |
| Multifamily | Pass                                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 9,094,426     | 9,114,466     |
| Multifamily | Special Mention                                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 169,933       | 249,165       |
| Multifamily | Substandard                                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 65,717        | 74,410        |
| Multifamily | Doubtful                                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 406           | 422           |
| Remaining commercial                                                                                            |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 2,739,515     | 2,676,506     |
| Remaining commercial | Pass                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 2,702,806     | 2,631,935     |
| Remaining commercial | Special Mention                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 12,441        | 28,686        |
| Remaining commercial | Substandard                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 23,698        | 15,601        |
| Remaining commercial | Doubtful                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 570           | 284           |
| Commercial | Corporate banking                                                                                  |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 2,959,176     | 2,949,089     |
| Commercial | Corporate banking | Pass                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 2,675,140     | 2,627,159     |
| Commercial | Corporate banking | Special Mention                                                                |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 80,161        | 99,090        |
| Commercial | Corporate banking | Substandard                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 190,974       | 208,785       |
| Commercial | Corporate banking | Doubtful                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 12,901        | 14,055        |
| Commercial | Middle market commercial real estate                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 4,855,085     | 4,223,359     |
| Commercial | Middle market commercial real estate | Pass                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 4,621,615     | 4,055,622     |
| Commercial | Middle market commercial real estate | Special Mention                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 80,253        | 29,621        |
| Commercial | Middle market commercial real estate | Substandard                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 98,361        | 117,571       |
| Commercial | Middle market commercial real estate | Doubtful                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 54,856        | 20,545        |
| Commercial | Santander real estate capital                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 1,501,030     | 1,550,469     |
| Commercial | Santander real estate capital | Pass                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 1,401,131     | 1,363,031     |
| Commercial | Santander real estate capital | Special Mention                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 71,246        | 144,597       |
| Commercial | Santander real estate capital | Substandard                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | 28,653        | 42,187        |
| Commercial | Santander real estate capital | Doubtful                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                      |          |          |
| Commercial loans                                                                                                | $ 0           | $ 654         |

| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans and Leases Receivable, Including Held-for-Sale, Commercial, Net of Deferred Income
+ References
No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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No definition available.
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