| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Troubled Debt Restructuring Activity) (Details) $ in Thousands** | 3 Months Ended | 6 Months Ended |               |
|-----------------------------------------------------------------------------------------------------------|----------------|----------------|---------------|
||
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Troubled debt restructurings                                                                              | $ 5,135,965    |           | $ 5,135,965   |   | $ 4,399,546 |
| Number of Contracts | contract                                                                            | 49,721         | 75,439         | 109,192       | 155,580 |   |
| Pre-TDR Recorded Investment                                                                               | $ 965,979      | $ 1,099,688    | $ 1,754,928   | $ 2,255,980 |   |
| Post-TDR Recorded Investment                                                                              | $ (979,853)    | $ (1,100,879)  | $ (1,757,113) | $ (2,251,865) |   |
| Number of Contracts | contract                                                                            | 11,265         | 13,340         | 26,061        | 24,435 |   |
| Recorded Investment                                                                                       | $ 166,744      | $ 187,503      | $ 387,450     | $ 333,161 |   |
| Performing                                                                                                |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Troubled debt restructurings                                                                              | 4,429,024      |           | 4,429,024     |   | 3,797,231 |
| Non-performing                                                                                            |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Troubled debt restructurings                                                                              | $ 706,941      |           | $ 706,941     |   | $ 602,315 |
| Commercial | Corporate banking                                                                            |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 10             | 10             | 19            | 12 |   |
| Pre-TDR Recorded Investment                                                                               | $ 104,123      | $ 10,225       | $ 151,899     | $ 11,673 |   |
| Post-TDR Recorded Investment                                                                              | $ (88,558)     | $ (11,788)     | $ (125,724)   | $ (13,227) |   |
| Commercial | Middle market commercial real estate                                                         |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 1              | 0              | 4             | 1 |   |
| Pre-TDR Recorded Investment                                                                               | $ 372          | $ 0            | $ 10,826      | $ 14,439 |   |
| Post-TDR Recorded Investment                                                                              | $ (30,317)     | $ 0            | $ (40,702)    | $ (14,439) |   |
| Commercial | Santander real estate capital                                                                |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts                                                                                       | 1              | 0              | 1             |   |   |
| Pre-TDR Recorded Investment                                                                               | $ 8,729        | $ 0            | $ 8,729       |   |   |
| Post-TDR Recorded Investment                                                                              | $ (8,711)      | $ 0            | $ (8,711)     |   |   |
| Commercial | Commercial and industrial                                                                    |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts                                                                                       | 337            | 127            | 560           | 208 |   |
| Pre-TDR Recorded Investment                                                                               | $ 9,839        | $ 4,251        | $ 17,069      | $ 15,124 |   |
| Post-TDR Recorded Investment                                                                              | $ (9,838)      | $ (4,251)      | $ (17,067)    | $ (12,410) |   |
| Number of Contracts | contract                                                                            | 35             | 12             | 90            | 22 |   |
| Recorded Investment                                                                                       | $ 1,068        | $ 410          | $ 3,068       | $ 686 |   |
| Consumer                                                                                                  |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            |           |           | 66,264        |   |   |
| Pre-TDR Recorded Investment                                                                               |           |           | $ 1,224,951   |   |   |
| Post-TDR Recorded Investment                                                                              |           |           | $ (1,224,593) |   |   |
| Consumer | Residential mortgages                                                                          |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 43             | 43             | 92            | 89 |   |
| Pre-TDR Recorded Investment                                                                               | $ 5,627        | $ 6,305        | $ 12,275      | $ 14,171 |   |
| Post-TDR Recorded Investment                                                                              | $ (5,950)      | $ (7,149)      | $ (12,803)    | $ (14,737) |   |
| Number of Contracts | contract                                                                            | 6              | 5              | 17            | 13 |   |
| Recorded Investment                                                                                       | $ 780          | $ 890          | $ 2,592       | $ 1,820 |   |
| Consumer | Home equity loans and lines of credit                                                          |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 35             | 22             | 102           | 61 |   |
| Pre-TDR Recorded Investment                                                                               | $ 2,504        | $ 1,023        | $ 7,043       | $ 4,694 |   |
| Post-TDR Recorded Investment                                                                              | $ (2,455)      | $ (1,617)      | $ (6,889)     | $ (5,288) |   |
| Number of Contracts | contract                                                                            | 3              | 6              | 8             | 11 |   |
| Recorded Investment                                                                                       | $ 155          | $ 674          | $ 496         | $ 937 |   |
| Consumer | Retail installment contracts and auto loans - originated                                       |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 38,569         | 19,495         |          | 26,526 |   |
| Pre-TDR Recorded Investment                                                                               | $ 710,885      | $ 371,937      |          | $ 509,687 |   |
| Post-TDR Recorded Investment                                                                              | $ (710,671)    | $ (371,830)    |          | $ (509,509) |   |
| Consumer | Retail installment contracts and auto loans - purchased                                        |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 10,501         | 51,049         | 24,845        | 119,507 |   |
| Pre-TDR Recorded Investment                                                                               | $ 122,739      | $ 700,250      | $ 298,707     | $ 1,674,181 |   |
| Post-TDR Recorded Investment                                                                              | $ (122,214)    | $ (698,541)    | $ (297,524)   | $ (1,670,284) |   |
| Consumer | Retail installment contracts and auto loans - originated                                       |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 9,944          | 12,143         | 23,068        | 21,836 |   |
| Recorded Investment                                                                                       | $ 163,142      | $ 184,270      | $ 377,844     | $ 326,804 |   |
| Consumer | Personal unsecured loans                                                                       |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 220            | 4,691          | 17,275        | 9,159 |   |
| Pre-TDR Recorded Investment                                                                               | $ 994          | $ 5,486        | $ 22,339      | $ 10,880 |   |
| Post-TDR Recorded Investment                                                                              | $ (973)        | $ (5,459)      | $ (22,189)    | $ (10,815) |   |
| Number of Contracts | contract                                                                            | 1,277          | 1,174          | 2,876         | 2,551 |   |
| Recorded Investment                                                                                       | $ 1,599        | $ 1,259        | $ 3,426       | $ 2,670 |   |
| Consumer | Other consumer                                                                                 |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Number of Contracts | contract                                                                            | 4              | 2              | 30            | 17 |   |
| Pre-TDR Recorded Investment                                                                               | $ 167          | $ 211          | $ 1,090       | $ 1,131 |   |
| Post-TDR Recorded Investment                                                                              | $ (166)        | $ (244)        | $ (911)       | $ (1,156) |   |
| Number of Contracts | contract                                                                            | 0              | 0              | 2             | 2 |   |
| Recorded Investment                                                                                       | $ 0            | $ 0            | $ 24          | $ 244 |   |
| Term Extension                                                                                            |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (725)          | (2,404)        | (1,292)       | (4,233) |   |
| Term Extension | Commercial | Corporate banking                                                           |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (16)           | (993)          | (32)          | (993) |   |
| Term Extension | Commercial | Middle market commercial real estate                                        |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Term Extension | Commercial | Santander real estate capital                                               |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             |   |   |
| Term Extension | Commercial | Commercial and industrial                                                   |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (1)            | 0              | (2)           | (34) |   |
| Term Extension | Consumer                                                                                 |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              |           |           | (247)         |   |   |
| Term Extension | Consumer | Residential mortgages                                                         |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | (1)           | 0 |   |
| Term Extension | Consumer | Home equity loans and lines of credit                                         |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 132           | 0 |   |
| Term Extension | Consumer | Retail installment contracts and auto loans - originated                      |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (164)          | (37)           |          | (51) |   |
| Term Extension | Consumer | Retail installment contracts and auto loans - purchased                       |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (544)          | (1,374)        | (1,142)       | (3,155) |   |
| Term Extension | Consumer | Personal unsecured loans                                                      |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Term Extension | Consumer | Other consumer                                                                |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Principal Forbearance                                                                                     |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (15,273)       | (4)            | (25,856)      | (2,708) |   |
| Principal Forbearance | Commercial | Corporate banking                                                    |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (15,273)       | (4)            | (25,856)      | (4) |   |
| Principal Forbearance | Commercial | Middle market commercial real estate                                 |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Principal Forbearance | Commercial | Santander real estate capital                                        |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             |   |   |
| Principal Forbearance | Commercial | Commercial and industrial                                            |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | (2,679) |   |
| Principal Forbearance | Consumer                                                                          |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              |           |           | 0             |   |   |
| Principal Forbearance | Consumer | Residential mortgages                                                  |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | (25) |   |
| Principal Forbearance | Consumer | Home equity loans and lines of credit                                  |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Principal Forbearance | Consumer | Retail installment contracts and auto loans - originated               |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              |          | 0 |   |
| Principal Forbearance | Consumer | Retail installment contracts and auto loans - purchased                |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Principal Forbearance | Consumer | Personal unsecured loans                                               |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Principal Forbearance | Consumer | Other consumer                                                         |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | 0 |   |
| Other                                                                                                     |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 29,872         | 3,599          | 29,333        | 2,826 |   |
| Other | Commercial | Corporate banking                                                                    |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (276)          | 2,560          | (287)         | 2,551 |   |
| Other | Commercial | Middle market commercial real estate                                                 |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 29,945         | 0              | 29,876        | 0 |   |
| Other | Commercial | Santander real estate capital                                                        |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (18)           | 0              | (18)          |   |   |
| Other | Commercial | Commercial and industrial                                                            |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 0              | 0              | 0             | (1) |   |
| Other | Consumer                                                                                          |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              |           |           | (111)         |   |   |
| Other | Consumer | Residential mortgages                                                                  |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 323            | 844            | 529           | 591 |   |
| Other | Consumer | Home equity loans and lines of credit                                                  |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (49)           | 594            | (286)         | 594 |   |
| Other | Consumer | Retail installment contracts and auto loans - originated                               |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (50)           | (70)           |          | (127) |   |
| Other | Consumer | Retail installment contracts and auto loans - purchased                                |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | 19             | (335)          | (41)          | (742) |   |
| Other | Consumer | Personal unsecured loans                                                               |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | (21)           | (27)           | (150)         | (65) |   |
| Other | Consumer | Other consumer                                                                         |           |           |          |   |   |
| **Financing Receivable, Modifications [Line Items]**                                                      |           |           |          |   |   |
| Post-TDR Recorded Investment                                                                              | $ (1)          | $ 33           | $ (179)       | $ 25 |   |

| 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=66008989&loc=SL6953676-111524
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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=66008989&loc=SL6953676-111524
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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=66008989&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=d3e18845-111554Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Recorded Investment -URI http://asc.fasb.org/extlink&oid=65880944
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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
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
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