| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (As Restated) (Impaired Loans) (Details) - USD ($)**  | 3 Months Ended | 12 Months Ended |
|-----------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Loans and Leases Receivable Disclosure [Abstract]**                                         |           |            |
| Minimum amount for commercial non-accrual loans                                               | $ 1,000,000    |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable, recorded investment                                            | 3,396,328,000  | $ 2,519,217,000 |
| Impaired financing receivable, unpaid principal balance                                       | 3,882,252,000  | 2,898,294,000   |
| Impaired financing receivable, related specific reserves                                      | 365,927,000    | 136,100,000     |
| Impaired financing receivables, average recorded investment                                   | 2,957,778,000  | 1,876,193,000   |
| Commercial [Member]                                                                           |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable, recorded investment                                            | 376,857,000    | 395,034,000     |
| Impaired financing receivable, unpaid principal balance                                       | 458,643,000    | 476,780,000     |
| Impaired financing receivable, related specific reserves                                      | 79,474,000     | 80,701,000      |
| Impaired financing receivables, average recorded investment                                   | 385,948,000    | 437,193,000     |
| Corporate banking [Member]                                                                    |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 36,933,000     | 37,735,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 58,426,000     | 59,950,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 39,807,000     | 40,453,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 68,522,000     | 66,328,000      |
| Impaired financing receivable, related specific reserves                                      | 22,569,000     | 25,322,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 37,334,000     | 40,610,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 59,188,000     | 56,856,000      |
| Middle market commercial real estate [Member]                                                 |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 111,944,000    | 127,792,000     |
| Impaired financing receivable with related allowance recorded, recorded investment            | 54,298,000     | 60,098,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 156,918,000    | 172,766,000     |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 60,224,000     | 66,024,000      |
| Impaired financing receivable, related specific reserves                                      | 16,114,000     | 17,004,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 119,868,000    | 114,465,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 57,198,000     | 89,472,000      |
| Santander real estate capital [Member]                                                        |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,936,000      | 2,982,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,761,000      | 3,878,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,936,000      | 2,982,000       |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 5,239,000      | 6,356,000       |
| Impaired financing receivable, related specific reserves                                      | 342,000        | 364,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 2,959,000      | 1,867,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 3,320,000      | 6,630,000       |
| Commercial and industrial loans [Member]                                                      |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 3,894,000      | 7,925,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 82,297,000     | 64,183,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 5,167,000      | 17,732,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 95,308,000     | 72,488,000      |
| Impaired financing receivable, related specific reserves                                      | 39,103,000     | 35,848,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 5,910,000      | 10,529,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 73,240,000     | 82,204,000      |
| Multi-family loans [Member]                                                                   |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 16,553,000     | 22,492,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 5,226,000      | 5,979,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 16,553,000     | 22,492,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 6,319,000      | 7,076,000       |
| Impaired financing receivable, related specific reserves                                      | 731,000        | 1,475,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 19,523,000     | 24,762,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 5,603,000      | 8,699,000       |
| Other commercial [Member]                                                                     |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 0              | 88,000          |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,589,000      | 1,932,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 0              | 88,000          |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 1,650,000      | 1,995,000       |
| Impaired financing receivable, related specific reserves                                      | 615,000        | 688,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 44,000         | 44,000          |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,761,000      | 1,055,000       |
| Consumer [Member]                                                                             |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 9,329,000      | 5,600,000       |
| Impaired financing receivable, recorded investment                                            | 3,019,471,000  | 2,124,183,000   |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 11,148,000     | 5,600,000       |
| Impaired financing receivable, unpaid principal balance                                       | 3,423,609,000  | 2,421,514,000   |
| Impaired financing receivable, related specific reserves                                      | 286,453,000    | 55,399,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 7,465,000      | 6,973,000       |
| Impaired financing receivables, average recorded investment                                   | 2,571,830,000  | 1,439,000,000   |
| Residential mortgages [Member]                                                                |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 21,725,000     | 23,408,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 136,201,000    | 130,813,000     |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 21,725,000     | 23,408,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 162,550,000    | 156,669,000     |
| Impaired financing receivable, related specific reserves                                      | 25,701,000     | 23,628,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 22,567,000     | 57,776,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 133,507,000    | 339,071,000     |
| Home equity loans and lines of credit [Member]                                                |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 26,441,000     | 27,230,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 63,224,000     | 60,132,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 26,441,000     | 27,230,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 72,651,000     | 69,374,000      |
| Impaired financing receivable, related specific reserves                                      | 4,580,000      | 5,002,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 26,836,000     | 29,152,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 61,678,000     | 57,516,000      |
| Retail installment contracts and auto loans, originated [Member]                              |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         |           | 0               |
| Impaired financing receivable with related allowance recorded, recorded investment            | 188,748,000    | 56,513,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    |           | 0               |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 194,337,000    | 58,229,000      |
| Impaired financing receivable, related specific reserves                                      | 57,310,000     | 16,997,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 0              | 0               |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 122,631,000    | 28,258,000      |
| Retail installment contracts and auto loans, purchased [Member]                               |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         |           | 1,787,124,000   |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,396,148,000  | 0               |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    |           | 2,040,785,000   |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,708,060,000  | 0               |
| Impaired financing receivable, related specific reserves                                      | 187,615,000    | 0               |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 964,269,000    | 893,563,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,198,074,000  | 0               |
| Personal unsecured loan [Member]                                                              |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 509,000        | 592,000         |
| Impaired financing receivable with related allowance recorded, recorded investment            | 19,070,000     | 16,476,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 509,000        | 592,000         |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 19,385,000     | 16,815,000      |
| Impaired financing receivable, related specific reserves                                      | 7,519,000      | 6,508,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 551,000        | 296,000         |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 17,773,000     | 9,506,000       |
| Other consumer [Member]                                                                       |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with related allowance recorded, recorded investment            | 16,663,000     | 16,295,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 22,617,000     | 22,812,000      |
| Impaired financing receivable, related specific reserves                                      | 3,728,000      | 3,264,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | $ 16,479,000   | $ 16,889,000    |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum amount for commercial nonaccrual loans defined as impaired loans.
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| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
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Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
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Amount of allowance for credit losses related to recorded investment.
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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.
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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.
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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.
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Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.
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