| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Impaired Loans) (Details) - USD ($)**                | 3 Months Ended | 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                                            | 5,013,456,000  | $ 3,398,375,000 | $ 4,542,986,000 |
| Impaired financing receivable, unpaid principal balance                                       | 5,477,495,000  | 3,882,252,000   | 5,044,479,000 |
| Impaired financing receivable, related specific reserves                                      | 1,098,166,000  | 337,146,000     | 899,415,000 |
| Impaired financing receivables, average recorded investment                                   | 4,778,226,000  | 2,637,365,000   | 3,531,274,000 |
| Commercial                                                                                    |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable, recorded investment                                            | 564,134,000    | 376,865,000     | 327,720,000 |
| Impaired financing receivable, unpaid principal balance                                       | 631,497,000    | 458,643,000     | 415,273,000 |
| Impaired financing receivable, related specific reserves                                      | 129,304,000    | 79,032,000      | 51,743,000 |
| Impaired financing receivables, average recorded investment                                   | 445,928,000    | 407,028,000     | 361,380,000 |
| Commercial | Corporate banking                                                                |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 34,657,000     |            | 40,843,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 41,908,000     |            | 31,376,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 37,396,000     |            | 43,582,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 43,172,000     |            | 32,650,000 |
| Impaired financing receivable, related specific reserves                                      | 7,814,000      |            | 6,413,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 37,750,000     |            | 39,289,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 36,642,000     |            | 45,663,000 |
| Commercial | Middle market commercial real estate                                             |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 77,307,000     |            | 78,325,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 73,054,000     |            | 38,046,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 122,255,000    |            | 123,495,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 83,875,000     |            | 43,745,000 |
| Impaired financing receivable, related specific reserves                                      | 13,100,000     |            | 5,624,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 77,816,000     |            | 103,059,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 55,550,000     |            | 49,072,000 |
| Commercial | Santander real estate capital                                                    |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,754,000      |            | 2,815,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 0              |            | 654,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,754,000      |            | 2,815,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 0              |            | 782,000 |
| Impaired financing receivable, related specific reserves                                      | 0              |            | 98,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 2,785,000      |            | 2,899,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 327,000        |            | 2,266,000 |
| Commercial | Commercial and industrial loans                                                  |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 5,360,000      | 5,132,000       | 3,635,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 310,467,000    | 81,067,000      | 113,451,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 6,636,000      | 6,433,000       | 5,046,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 315,312,000    | 94,042,000      | 142,308,000 |
| Impaired financing receivable, related specific reserves                                      | 108,047,000    | 38,661,000      | 38,416,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 4,498,000      | 7,830,000       | 5,780,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 211,959,000    | 81,636,000      | 88,818,000 |
| Commercial | Multifamily loans                                                                |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 13,525,000     |            | 9,467,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 4,285,000      |            | 5,653,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 14,537,000     |            | 10,488,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 4,290,000      |            | 5,658,000 |
| Impaired financing receivable, related specific reserves                                      | 233,000        |            | 443,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 11,496,000     |            | 15,980,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 4,969,000      |            | 5,816,000 |
| Commercial | Other commercial                                                                 |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 193,000        |            | 239,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 624,000        |            | 3,216,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 193,000        |            | 239,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 1,077,000      |            | 4,465,000 |
| Impaired financing receivable, related specific reserves                                      | 110,000        |            | 749,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 216,000        |            | 164,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,920,000      |            | 2,574,000 |
| Consumer                                                                                      |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable, recorded investment                                            | 4,449,322,000  | 3,021,510,000   | 4,215,266,000 |
| Impaired financing receivable, unpaid principal balance                                       | 4,845,998,000  | 3,423,609,000   | 4,629,206,000 |
| Impaired financing receivable, related specific reserves                                      | 968,862,000    | 258,114,000     | 847,672,000 |
| Impaired financing receivables, average recorded investment                                   | 4,332,298,000  | 2,230,337,000   | 3,169,894,000 |
| Consumer | Residential mortgages                                                              |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 68,860,000     |            | 26,808,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 131,665,000    |            | 172,265,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 68,860,000     |            | 26,808,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 159,330,000    |            | 200,176,000 |
| Impaired financing receivable, related specific reserves                                      | 25,157,000     |            | 25,034,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 47,834,000     |            | 25,108,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 151,965,000    |            | 151,539,000 |
| Consumer | Home equity loans and lines of credit                                              |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 52,211,000     |            | 31,080,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 51,512,000     |            | 71,847,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 52,211,000     |            | 31,080,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 66,348,000     |            | 86,355,000 |
| Impaired financing receivable, related specific reserves                                      | 4,242,000      |            | 3,757,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 41,646,000     |            | 29,155,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 61,680,000     |            | 65,990,000 |
| Consumer | Retail installment contracts and auto loans                                        |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 76,371,000     | 146,136,000     | 87,138,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 4,026,246,000  | 2,586,190,000   | 3,790,551,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 96,361,000     | 189,983,000     | 110,575,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 4,354,771,000  | 2,902,397,000   | 4,133,121,000 |
| Impaired financing receivable, related specific reserves                                      | 936,010,000    | 216,586,000     | 815,226,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 81,755,000     | 519,849,000     | 937,131,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 3,908,401,000  | $ 1,307,306,000 | 1,923,700,000 |
| Consumer | Personal unsecured loans                                                           |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 19,775,000     |            | 12,865,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,607,000      |            | 1,839,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 19,775,000     |            | 12,865,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,838,000      |            | 2,226,000 |
| Impaired financing receivable, related specific reserves                                      | 421,000        |            | 430,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 16,320,000     |            | 6,729,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,723,000      |            | 9,158,000 |
| Consumer | Other consumer                                                                     |           |            |   |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |   |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 6,508,000      |            | 2,210,000 |
| Impaired financing receivable with related allowance recorded, recorded investment            | 14,567,000     |            | 18,663,000 |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 6,508,000      |            | 2,210,000 |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 18,996,000     |            | 23,790,000 |
| Impaired financing receivable, related specific reserves                                      | 3,032,000      |            | 3,225,000 |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 4,359,000      |            | 3,905,000 |
| Impaired financing receivable with related allowance recorded, average recorded investment    | $ 16,615,000   |            | $ 17,479,000 |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum amount for commercial nonaccrual loans defined as impaired loans.
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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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| - 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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Amount of investment in impaired financing receivables for which there is a 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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