| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Impaired Loans) (Details) - USD ($)**                | 6 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                                            | 5,372,931,000  | $ 4,531,285,000 |
| Impaired financing receivable, unpaid principal balance                                       | 5,840,682,000  | 5,044,479,000   |
| Impaired financing receivable, related specific reserves                                      | 1,355,832,000  | 944,091,000     |
| Impaired financing receivables, average recorded investment                                   | 4,952,115,000  | 3,525,256,000   |
| Commercial                                                                                    |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable, recorded investment                                            | 559,414,000    | 327,627,000     |
| Impaired financing receivable, unpaid principal balance                                       | 625,531,000    | 415,273,000     |
| Impaired financing receivable, related specific reserves                                      | 129,084,000    | 48,511,000      |
| Impaired financing receivables, average recorded investment                                   | 443,525,000    | 361,333,000     |
| Commercial | Corporate banking                                                                |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 36,629,000     | 40,843,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 34,575,000     | 31,376,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 36,860,000     | 43,582,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 36,327,000     | 32,650,000      |
| Impaired financing receivable, related specific reserves                                      | 9,103,000      | 6,413,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 38,736,000     | 39,289,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 32,976,000     | 45,663,000      |
| Commercial | Middle market commercial real estate                                             |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 61,467,000     | 78,325,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 73,941,000     | 38,046,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 106,415,000    | 123,495,000     |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 84,730,000     | 43,745,000      |
| Impaired financing receivable, related specific reserves                                      | 12,954,000     | 5,624,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 69,896,000     | 103,059,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 55,994,000     | 49,072,000      |
| Commercial | Santander real estate capital                                                    |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 2,709,000      | 2,815,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 8,711,000      | 654,000         |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 2,709,000      | 2,815,000       |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 8,711,000      | 782,000         |
| Impaired financing receivable, related specific reserves                                      | 950,000        | 98,000          |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 2,762,000      | 2,899,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 4,683,000      | 2,266,000       |
| Commercial | Commercial and industrial loans                                                  |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 40,442,000     | 3,635,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 278,813,000    | 113,358,000     |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 41,755,000     | 5,046,000       |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 285,073,000    | 142,308,000     |
| Impaired financing receivable, related specific reserves                                      | 104,864,000    | 35,184,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 22,039,000     | 5,780,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 196,086,000    | 88,771,000      |
| Commercial | Multifamily loans                                                                |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 12,698,000     | 9,467,000       |
| Impaired financing receivable with related allowance recorded, recorded investment            | 966,000        | 5,653,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 13,474,000     | 10,488,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 971,000        | 5,658,000       |
| Impaired financing receivable, related specific reserves                                      | 653,000        | 443,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 11,083,000     | 15,980,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 3,310,000      | 5,816,000       |
| Commercial | Other commercial                                                                 |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 908,000        | 239,000         |
| Impaired financing receivable with related allowance recorded, recorded investment            | 7,555,000      | 3,216,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 908,000        | 239,000         |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 7,598,000      | 4,465,000       |
| Impaired financing receivable, related specific reserves                                      | 560,000        | 749,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 574,000        | 164,000         |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 5,386,000      | 2,574,000       |
| Consumer                                                                                      |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable, recorded investment                                            | 4,813,517,000  | 4,203,658,000   |
| Impaired financing receivable, unpaid principal balance                                       | 5,215,151,000  | 4,629,206,000   |
| Impaired financing receivable, related specific reserves                                      | 1,226,748,000  | 895,580,000     |
| Impaired financing receivables, average recorded investment                                   | 4,508,590,000  | 3,163,923,000   |
| Consumer | Residential mortgages                                                              |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 66,915,000     | 26,808,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 131,971,000    | 172,265,000     |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 66,915,000     | 26,808,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 157,281,000    | 200,176,000     |
| Impaired financing receivable, related specific reserves                                      | 27,185,000     | 25,034,000      |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 46,862,000     | 25,108,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 152,118,000    | 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         | 52,357,000     | 31,080,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 51,110,000     | 71,847,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 52,357,000     | 31,080,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 65,749,000     | 86,355,000      |
| Impaired financing receivable, related specific reserves                                      | 4,797,000      | 3,757,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 41,719,000     | 29,155,000      |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 61,479,000     | 65,990,000      |
| Consumer | Retail installment contracts and auto loans - originated                           |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 9,000          | 15,000          |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,218,319,000  | 1,325,975,000   |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 9,000          | 15,000          |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,269,720,000  | 1,359,585,000   |
| Impaired financing receivable, related specific reserves                                      | 690,186,000    | 408,208,000     |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 12,000         | 8,000           |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,772,147,000  | 691,244,000     |
| Consumer | Retail installment contracts and auto loans - purchased                            |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 49,886,000     | 75,698,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 2,185,838,000  | 2,454,108,000   |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 65,993,000     | 96,768,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 2,470,349,000  | 2,773,536,000   |
| Impaired financing receivable, related specific reserves                                      | 501,109,000    | 454,926,000     |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 62,792,000     | 931,411,000     |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 2,319,973,000  | 1,227,054,000   |
| Consumer | Personal unsecured loans                                                           |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 21,742,000     | 12,865,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 1,585,000      | 1,839,000       |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 21,742,000     | 12,865,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 1,897,000      | 2,226,000       |
| Impaired financing receivable, related specific reserves                                      | 385,000        | 430,000         |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 17,304,000     | 6,729,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | 1,712,000      | 9,158,000       |
| Consumer | Other consumer                                                                     |           |            |
| **Financing Receivable, Impaired [Line Items]**                                               |           |            |
| Impaired financing receivable with no related allowance recorded, recorded investment         | 19,850,000     | 12,495,000      |
| Impaired financing receivable with related allowance recorded, recorded investment            | 13,935,000     | 18,663,000      |
| Impaired financing receivable with no related allowance recorded, unpaid principal balance    | 24,213,000     | 16,002,000      |
| Impaired financing receivable with related allowance recorded, unpaid principal balance       | 18,926,000     | 23,790,000      |
| Impaired financing receivable, related specific reserves                                      | 3,086,000      | 3,225,000       |
| Impaired financing receivable with no related allowance recorded, average recorded investment | 16,173,000     | 9,048,000       |
| Impaired financing receivable with related allowance recorded, average recorded investment    | $ 16,299,000   | $ 17,479,000    |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Minimum amount for commercial nonaccrual loans defined as impaired loans.
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Amount of allowance for credit losses related to recorded investment.
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