| **LOANS AND ALLOWANCE FOR CREDIT LOSSES - Impaired Loans by Class (Details) (USD $)In Thousands, unless otherwise specified**                                                         | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | $ 317,097       |  | $ 360,815 |  |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 916,055         |  | 1,029,392 |  |
| Total impaired loans                                                                                                                                                                  | 1,233,152       |  | 1,390,207 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 225,796         |  | 267,054 |  |
| Corporate Banking [Member]                                                                                                                                                            |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 54,637          | [1] | 67,261 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 154,810         | [1] | 206,253 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 76,139          |  | 89,023 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 169,511         |  | 226,594 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 60,892          |  | 59,740 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 60,949          |  | 54,950 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 180,532         |  | 204,842 |  |
| Middle Market Commercial Real Estate [Member]                                                                                                                                         |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 103,670         | [1] | 103,872 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 120,387         | [1] | 153,978 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 125,933         |  | 104,486 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 184,672         |  | 231,960 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 26,130          |  | 37,393 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 103,771         |  | 116,437 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 137,183         |  | 227,824 |  |
| Santander Real Estate Capital [Member]                                                                                                                                                |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 24,919          | [1] | 24,118 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 18,611          | [1] | 70,323 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 24,919          |  | 24,118 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 18,611          |  | 79,116 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 3,572           |  | 15,651 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 24,519          |  | 23,921 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 44,467          |  | 93,713 |  |
| Remaining Commercial [Member]                                                                                                                                                         |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 2,309           | [1] | 8,017 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 0               | [1] | 0 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 2,308           |  | 8,017 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 0               |  | 0 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 0               |  | 0 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 5,162           |  | 12,537 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 0               |  | 979 |  |
| Home Mortgages [Member]                                                                                                                                                               |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 92,143          | [1] | 105,604 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 547,329         | [1] | 529,966 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 92,143          |  | 105,604 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 603,866         |  | 584,571 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 124,746         |  | 138,629 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 98,874          |  | 52,802 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 538,648         |  | 512,199 |  |
| Self-originated Home Equity [Member]                                                                                                                                                  |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 31,074          | [1] | 39,950 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 54,900          | [1] |   |  |
| Total impaired loans                                                                                                                                                                  |            |  | 44,352 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 31,074          |  | 39,950 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 63,975          |  |   |  |
| Impaired financing receivable, unpaid principal balance                                                                                                                               |            |  | 56,691 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 5,479           |  | 7,532 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 35,512          |  | 39,136 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 49,626          |  | 22,176 |  |
| Indirect Purchased [Member]                                                                                                                                                           |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 818             | [1] | 1,404 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 2,279           |  | 3,231 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 1,111           |  | 702 |  |
| Credit Cards [Member]                                                                                                                                                                 |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 2,535           | [1] | 5,926 | [1] |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 2,535           |  | 5,926 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 677             |  | 3,420 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 4,231           |  | 2,963 |  |
| Remaining Consumer [Member]                                                                                                                                                           |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Impaired financing receivable with no related allowance recorded, recorded investment                                                                                                 | 7,527           | [1] | 10,589 | [1] |
| Impaired financing receivable with related allowance recorded, recorded investment                                                                                                    | 17,483          | [1] | 18,594 | [1] |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance                                                                                            | 15,141          |  | 14,588 |  |
| Impaired financing receivable with related allowance recorded, unpaid principal balance                                                                                               | 17,482          |  | 26,253 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 4,300           |  | 4,689 |  |
| Impaired financing receivable with no related allowance recorded, average recorded investment                                                                                         | 9,058           |  | 14,763 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            | 18,037          |  | 9,297 |  |
| Commercial [Member]                                                                                                                                                                   |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Total impaired loans                                                                                                                                                                  | 479,343         | [1] | 633,822 | [1] |
| Impaired financing receivable, unpaid principal balance                                                                                                                               | 602,093         |  | 763,314 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 90,594          |  | 112,784 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            |            |  | 735,203 |  |
| Impaired financing receivables, average recorded investment                                                                                                                           | 556,583         |  |   |  |
| Consumer [Member]                                                                                                                                                                     |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Total impaired loans                                                                                                                                                                  | 753,809         | [1] | 756,385 | [1] |
| Impaired financing receivable, unpaid principal balance                                                                                                                               | 828,495         |  | 836,814 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 135,202         |  | 154,270 |  |
| Impaired financing receivable with related allowance recorded, average recorded investment                                                                                            |            |  | 654,038 |  |
| Impaired financing receivables, average recorded investment                                                                                                                           | 755,097         |  |   |  |
| Total [Member]                                                                                                                                                                        |            |  |   |  |
| **Financing Receivable, Impaired [Line Items]**                                                                                                                                       |            |  |   |  |
| Total impaired loans                                                                                                                                                                  | 1,233,152       | [1] | 1,390,207 | [1] |
| Impaired financing receivable, unpaid principal balance                                                                                                                               | 1,430,588       |  | 1,600,128 |  |
| Impaired financing receivable, related specific reserves                                                                                                                              | 225,796         |  | 267,054 |  |
| Impaired financing receivables, average recorded investment                                                                                                                           | $ 1,311,680     |  | $ 1,389,241 |  |
|                                                                                                                                                                                       |
| [1]Recorded investment includes deferred loan fees, net of deferred origination costs and unamortized purchase premiums, net of discounts as well as purchase accounting adjustments. |

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