| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (As Restated) (Troubled Debt Restructuring Activity) (Details) $ in Thousands** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Troubled debt restructurings                                                                                            | $ 4,399,546     | $ 2,385,175 |
| Number of Contracts | contract                                                                                          | 271,505         | 157,407 |
| Pre-TDR Recorded Investment                                                                                             | $ 4,104,924     | $ 2,199,635 |
| Post-TDR Recorded Investment                                                                                            | (4,091,728)     | (2,185,939) |
| Performing                                                                                                              |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Troubled debt restructurings                                                                                            | 3,797,231       | 2,041,653 |
| Non-performing                                                                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Troubled debt restructurings                                                                                            | $ 602,315       | $ 343,522 |
| Commercial | Corporate banking                                                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 21              | 30 |
| Pre-TDR Recorded Investment                                                                                             | $ 40,931        | $ 73,246 |
| Post-TDR Recorded Investment                                                                                            | $ (36,994)      | $ (71,692) |
| Commercial | Middle market commercial real estate                                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 2               | 7 |
| Pre-TDR Recorded Investment                                                                                             | $ 17,024        | $ 70,353 |
| Post-TDR Recorded Investment                                                                                            | $ (14,914)      | $ (67,453) |
| Commercial | Santander real estate capital                                                                              |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | Contract                                                                                          | 1               |   |
| Pre-TDR Recorded Investment                                                                                             | $ 4,977         |   |
| Post-TDR Recorded Investment                                                                                            | $ (4,970)       |   |
| Commercial | Commercial and industrial                                                                                  |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts                                                                                                     | 595             | 72 |
| Pre-TDR Recorded Investment                                                                                             | $ 19,108        | $ 2,169 |
| Post-TDR Recorded Investment                                                                                            | $ (19,108)      | $ (2,166) |
| Number of Contracts | contract                                                                                          | 61              | 1 |
| Recorded Investment                                                                                                     | $ 1,961         | $ 52 |
| Commercial | Other commercial                                                                                           |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          |            | 5 |
| Pre-TDR Recorded Investment                                                                                             |            | $ 2,503 |
| Post-TDR Recorded Investment                                                                                            |            | $ (2,472) |
| Consumer                                                                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 54,964          | 8,841 |
| Recorded Investment                                                                                                     | $ 803,638       | $ 94,577 |
| Consumer | Residential mortgages                                                                                        |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 513             | 266 |
| Pre-TDR Recorded Investment                                                                                             | $ 69,563        | $ 47,643 |
| Post-TDR Recorded Investment                                                                                            | $ (69,158)      | $ (48,494) |
| Number of Contracts | contract                                                                                          | 22              | 28 |
| Recorded Investment                                                                                                     | $ 2,840         | $ 4,214 |
| Consumer | Home equity loans and lines of credit                                                                        |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 470             | 115 |
| Pre-TDR Recorded Investment                                                                                             | $ 31,848        | $ 10,509 |
| Post-TDR Recorded Investment                                                                                            | $ (31,592)      | $ (10,509) |
| Number of Contracts | contract                                                                                          | 15              | 9 |
| Recorded Investment                                                                                                     | $ 1,789         | $ 785 |
| Consumer | Retail installment contracts and auto loans, originated                                                      |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 78,576          | 2,871 |
| Pre-TDR Recorded Investment                                                                                             | $ 1,486,951     | $ 57,997 |
| Post-TDR Recorded Investment                                                                                            | $ (1,486,342)   | $ (57,904) |
| Consumer | Retail installment contracts and auto loans, purchased                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 175,780         | 140,029 |
| Pre-TDR Recorded Investment                                                                                             | $ 2,412,434     | $ 1,918,542 |
| Post-TDR Recorded Investment                                                                                            | $ (2,406,986)   | $ (1,908,674) |
| Consumer | Retail installment contracts and auto loans                                                                  |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 51,202          | 6,398 |
| Recorded Investment                                                                                                     | $ 792,721       | $ 87,019 |
| Consumer | Personal unsecured loans                                                                                     |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 15,492          | 13,999 |
| Pre-TDR Recorded Investment                                                                                             | $ 18,870        | $ 15,810 |
| Post-TDR Recorded Investment                                                                                            | $ (18,664)      | $ (15,683) |
| Number of Contracts | contract                                                                                          | 3,662           | 2,404 |
| Recorded Investment                                                                                                     | $ 4,083         | $ 2,480 |
| Consumer | Other consumer                                                                                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Number of Contracts | contract                                                                                          | 55              | 13 |
| Pre-TDR Recorded Investment                                                                                             | $ 3,218         | $ 863 |
| Post-TDR Recorded Investment                                                                                            | $ (3,000)       | $ (892) |
| Number of Contracts | contract                                                                                          | 2               | 1 |
| Recorded Investment                                                                                                     | $ 244           | $ 27 |
| Term Extension                                                                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (5,972)         | (7,914) |
| Term Extension | Commercial | Corporate banking                                                                         |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (1,027)         | (355) |
| Term Extension | Commercial | Middle market commercial real estate                                                      |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Term Extension | Commercial | Santander real estate capital                                                             |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               |   |
| Term Extension | Commercial | Commercial and industrial                                                                 |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | (3) |
| Term Extension | Commercial | Other commercial                                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            |            | (33) |
| Term Extension | Consumer | Residential mortgages                                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 10              | (18) |
| Term Extension | Consumer | Home equity loans and lines of credit                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Term Extension | Consumer | Retail installment contracts and auto loans, originated                                     |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (335)           | 0 |
| Term Extension | Consumer | Retail installment contracts and auto loans, purchased                                      |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (4,619)         | (7,504) |
| Term Extension | Consumer | Personal unsecured loans                                                                    |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Term Extension | Consumer | Other consumer                                                                              |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (1)             | (1) |
| Principal Forbearance                                                                                                   |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (3,506)         | (645) |
| Principal Forbearance | Commercial | Corporate banking                                                                  |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (2,716)         | (529) |
| Principal Forbearance | Commercial | Middle market commercial real estate                                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Principal Forbearance | Commercial | Santander real estate capital                                                      |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               |   |
| Principal Forbearance | Commercial | Commercial and industrial                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Principal Forbearance | Commercial | Other commercial                                                                   |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            |            | 2 |
| Principal Forbearance | Consumer | Residential mortgages                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (680)           | (118) |
| Principal Forbearance | Consumer | Home equity loans and lines of credit                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Principal Forbearance | Consumer | Retail installment contracts and auto loans, originated                              |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Principal Forbearance | Consumer | Retail installment contracts and auto loans, purchased                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Principal Forbearance | Consumer | Personal unsecured loans                                                             |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (110)           | 0 |
| Principal Forbearance | Consumer | Other consumer                                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Other                                                                                                                   |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (3,718)         | (5,137) |
| Other | Commercial | Corporate banking                                                                                  |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (194)           | (670) |
| Other | Commercial | Middle market commercial real estate                                                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (2,110)         | (2,900) |
| Other | Commercial | Santander real estate capital                                                                      |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (7)             |   |
| Other | Commercial | Commercial and industrial                                                                          |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 0               | 0 |
| Other | Commercial | Other commercial                                                                                   |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            |            | 0 |
| Other | Consumer | Residential mortgages                                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | 265             | 987 |
| Other | Consumer | Home equity loans and lines of credit                                                                |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (256)           | 0 |
| Other | Consumer | Retail installment contracts and auto loans, originated                                              |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (274)           | (93) |
| Other | Consumer | Retail installment contracts and auto loans, purchased                                               |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (829)           | (2,364) |
| Other | Consumer | Personal unsecured loans                                                                             |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | (96)            | (127) |
| Other | Consumer | Other consumer                                                                                       |            |   |
| **Financing Receivable, Modifications [Line Items]**                                                                    |            |   |
| Post-TDR Recorded Investment                                                                                            | $ (217)         | $ 30 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Post-modification amount of investment in financing receivables modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524
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