| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (As Restated) (Impaired Loans) (Details) - USD ($)**  | 12 Months Ended |
|-----------------------------------------------------------------------------------------------|-----------------|
||
| **Loans and Leases Receivable Disclosure [Abstract]**                                         |            |   |
| Minimum amount for commercial non-accrual loans (in excess of)                                | $ 1,000,000     |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable, recorded investment                                            | 4,531,285,000   | $ 2,519,217,000 |
| Impaired financing receivable, unpaid principal balance                                       | 5,044,479,000   | 2,898,294,000 |
| Impaired financing receivable, related specific reserves                                      | 944,091,000     | 136,100,000 |
| Impaired financing receivables, average recorded investment                                   | 3,525,256,000   | 1,876,193,000 |
| Commercial                                                                                    |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable, recorded investment                                            | 327,627,000     | 395,034,000 |
| Impaired financing receivable, unpaid principal balance                                       | 415,273,000     | 476,780,000 |
| Impaired financing receivable, related specific reserves                                      | 48,511,000      | 80,701,000 |
| Impaired financing receivables, average recorded investment                                   | 361,333,000     | 437,193,000 |
| Commercial | Corporate banking                                                                |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 40,843,000      | 37,735,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 31,376,000      | 59,950,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 43,582,000      | 40,453,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 32,650,000      | 66,328,000 |
| Impaired financing receivable, related specific reserves                                      | 6,413,000       | 25,322,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 39,289,000      | 40,610,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 45,663,000      | 56,856,000 |
| Commercial | Middle market commercial real estate                                             |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 78,325,000      | 127,792,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 38,046,000      | 60,098,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 123,495,000     | 172,766,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 43,745,000      | 66,024,000 |
| Impaired financing receivable, related specific reserves                                      | 5,624,000       | 17,004,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 103,059,000     | 114,465,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 49,072,000      | 89,472,000 |
| Commercial | Santander real estate capital                                                    |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,815,000       | 2,982,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 654,000         | 3,878,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,815,000       | 2,982,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 782,000         | 6,356,000 |
| Impaired financing receivable, related specific reserves                                      | 98,000          | 364,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 2,899,000       | 1,867,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 2,266,000       | 6,630,000 |
| Commercial | Commercial and industrial loans                                                  |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 3,635,000       | 7,925,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 113,358,000     | 64,183,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 5,046,000       | 17,732,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 142,308,000     | 72,488,000 |
| Impaired financing receivable, related specific reserves                                      | 35,184,000      | 35,848,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 5,780,000       | 10,529,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 88,771,000      | 82,204,000 |
| Commercial | Multifamily loans                                                                |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 9,467,000       | 22,492,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 5,653,000       | 5,979,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 10,488,000      | 22,492,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 5,658,000       | 7,076,000 |
| Impaired financing receivable, related specific reserves                                      | 443,000         | 1,475,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 15,980,000      | 24,762,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 5,816,000       | 8,699,000 |
| Commercial | Other commercial                                                                 |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 239,000         | 88,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 3,216,000       | 1,932,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 239,000         | 88,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 4,465,000       | 1,995,000 |
| Impaired financing receivable, related specific reserves                                      | 749,000         | 688,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 164,000         | 44,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 2,574,000       | 1,055,000 |
| Consumer                                                                                      |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable, recorded investment                                            | 4,203,658,000   | 2,124,183,000 |
| Impaired financing receivable, unpaid principal balance                                       | 4,629,206,000   | 2,421,514,000 |
| Impaired financing receivable, related specific reserves                                      | 895,580,000     | 55,399,000 |
| Impaired financing receivables, average recorded investment                                   | 3,163,923,000   | 1,439,000,000 |
| Consumer | Residential mortgages                                                              |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 26,808,000      | 23,408,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 172,265,000     | 130,813,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 26,808,000      | 23,408,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 200,176,000     | 156,669,000 |
| Impaired financing receivable, related specific reserves                                      | 25,034,000      | 23,628,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 25,108,000      | 57,776,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 151,539,000     | 339,071,000 |
| Consumer | Home equity loans and lines of credit                                              |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 31,080,000      | 27,230,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 71,847,000      | 60,132,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 31,080,000      | 27,230,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 86,355,000      | 69,374,000 |
| Impaired financing receivable, related specific reserves                                      | 3,757,000       | 5,002,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 29,155,000      | 29,152,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 65,990,000      | 57,516,000 |
| Consumer | Retail installment contracts and auto loans, originated                            |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 15,000          | 0 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,325,975,000   | 56,513,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 15,000          | 0 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 1,359,585,000   | 58,229,000 |
| Impaired financing receivable, related specific reserves                                      | 408,208,000     | 16,997,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 8,000           | 0 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 691,244,000     | 28,258,000 |
| Consumer | Retail installment contracts and auto loans, purchased                             |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 75,698,000      | 1,787,124,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,454,108,000   | 0 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 96,768,000      | 2,040,785,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,773,536,000   | 0 |
| Impaired financing receivable, related specific reserves                                      | 454,926,000     | 0 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 931,411,000     | 893,563,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,227,054,000   | 0 |
| Consumer | Personal unsecured loans                                                           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 12,865,000      | 592,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,839,000       | 16,476,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 12,865,000      | 592,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,226,000       | 16,815,000 |
| Impaired financing receivable, related specific reserves                                      | 430,000         | 6,508,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 6,729,000       | 296,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 9,158,000       | 9,506,000 |
| Consumer | Other consumer                                                                     |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 12,495,000      | 5,600,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 18,663,000      | 16,295,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 16,002,000      | 5,600,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 23,790,000      | 22,812,000 |
| Impaired financing receivable, related specific reserves                                      | 3,225,000       | 3,264,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 9,048,000       | 6,973,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | $ 17,479,000    | $ 16,889,000 |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Minimum amount for commercial nonaccrual loans defined as impaired loans.
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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 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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