| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Age Analysis of Past Due Loans) (Details) - USD ($) $ in Thousands** | 3 Months Ended |             |
|---------------------------------------------------------------------------------------------------------------|----------------|-------------|
||
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Percentage of payment needed on past due loans for qualification                                              | 90.00%         |        |
| Total Past Due                                                                                                | $ 3,670,110    | $ 4,455,024 |
| Current                                                                                                       | 80,086,838     | 78,219,548  |
| Total Financing Receivables                                                                                   | 83,756,948     | 82,674,572  |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 72,878         | 79,346      |
| 30-89 Days Past Due                                                                                           |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 3,041,627      | 3,697,443   |
| Greater Than 90 Days                                                                                          |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | $ 628,483      | 757,581     |
| Retail installment contracts and auto loans                                                                   |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Percentage of payment needed on past due loans for qualification                                              | 50.00%         |        |
| Commercial | Corporate banking                                                                                |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | $ 50,616       | 48,085      |
| Current                                                                                                       | 2,908,560      | 2,901,004   |
| Total Financing Receivables                                                                                   | 2,959,176      | 2,949,089   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Corporate banking | 30-89 Days Past Due                                                          |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 18,862         | 18,085      |
| Commercial | Corporate banking | Greater Than 90 Days                                                         |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 31,754         | 30,000      |
| Commercial | Middle market commercial real estate                                                             |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 57,978         | 21,638      |
| Current                                                                                                       | 4,797,107      | 4,201,721   |
| Total Financing Receivables                                                                                   | 4,855,085      | 4,223,359   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Middle market commercial real estate | 30-89 Days Past Due                                       |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 39,964         | 575         |
| Commercial | Middle market commercial real estate | Greater Than 90 Days                                      |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 18,014         | 21,063      |
| Commercial | Santander real estate capital                                                                    |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 10,240         | 654         |
| Current                                                                                                       | 1,490,790      | 1,549,815   |
| Total Financing Receivables                                                                                   | 1,501,030      | 1,550,469   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Santander real estate capital | 30-89 Days Past Due                                              |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 10,240         | 0           |
| Commercial | Santander real estate capital | Greater Than 90 Days                                             |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 0              | 654         |
| Commercial | Commercial and industrial loans                                                                  |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 93,426         | 75,589      |
| Current                                                                                                       | 20,240,821     | 19,796,256  |
| Total Financing Receivables                                                                                   | 20,334,247     | 19,871,845  |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Commercial and industrial loans | 30-89 Days Past Due                                            |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 54,972         | 31,488      |
| Commercial | Commercial and industrial loans | Greater Than 90 Days                                           |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 38,454         | 44,101      |
| Commercial | Multifamily                                                                                      |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 2,401          | 7,488       |
| Current                                                                                                       | 9,328,081      | 9,430,975   |
| Total Financing Receivables                                                                                   | 9,330,482      | 9,438,463   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Multifamily | 30-89 Days Past Due                                                                |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 2,117          | 2,951       |
| Commercial | Multifamily | Greater Than 90 Days                                                               |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 284            | 4,537       |
| Commercial | Other commercial                                                                                 |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 8,167          | 6,047       |
| Current                                                                                                       | 2,731,348      | 2,670,459   |
| Total Financing Receivables                                                                                   | 2,739,515      | 2,676,506   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Commercial | Other commercial | 30-89 Days Past Due                                                           |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 7,185          | 3,968       |
| Commercial | Other commercial | Greater Than 90 Days                                                          |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 982            | 2,079       |
| Consumer | Residential mortgages                                                                              |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 261,063        | 282,833     |
| Current                                                                                                       | 6,178,055      | 6,184,922   |
| Total Financing Receivables                                                                                   | 6,439,118      | 6,467,755   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Consumer | Residential mortgages | 30-89 Days Past Due                                                        |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 122,560        | 140,323     |
| Consumer | Residential mortgages | Greater Than 90 Days                                                       |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 138,503        | 142,510     |
| Consumer | Home equity loans and lines of credit                                                              |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 106,080        | 107,881     |
| Current                                                                                                       | 5,997,764      | 6,039,032   |
| Total Financing Receivables                                                                                   | 6,103,844      | 6,146,913   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Consumer | Home equity loans and lines of credit | 30-89 Days Past Due                                        |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 28,548         | 28,166      |
| Consumer | Home equity loans and lines of credit | Greater Than 90 Days                                       |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 77,532         | 79,715      |
| Consumer | Retail installment contracts and auto loans                                                        |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 2,868,108      | 3,671,736   |
| Current                                                                                                       | 23,981,170     | 21,997,496  |
| Total Financing Receivables                                                                                   | 26,849,278     | 25,669,232  |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Consumer | Retail installment contracts and auto loans | 30-89 Days Past Due                                  |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 2,647,084      | 3,354,728   |
| Consumer | Retail installment contracts and auto loans | Greater Than 90 Days                                 |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 221,024        | 317,008     |
| Consumer | Personal unsecured loans                                                                           |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 157,051        | 162,427     |
| Current                                                                                                       | 1,515,096      | 2,485,934   |
| Total Financing Receivables                                                                                   | 1,672,147      | 2,648,361   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 72,878         | 79,346      |
| Consumer | Personal unsecured loans | 30-89 Days Past Due                                                     |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 79,949         | 78,741      |
| Consumer | Personal unsecured loans | Greater Than 90 Days                                                    |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 77,102         | 83,686      |
| Consumer | Other consumer                                                                                     |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 54,980         | 70,646      |
| Current                                                                                                       | 918,046        | 961,934     |
| Total Financing Receivables                                                                                   | 973,026        | 1,032,580   |
| Recorded Investment Greater than 90 Days and Accruing                                                         | 0              | 0           |
| Consumer | Other consumer | 30-89 Days Past Due                                                               |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | 30,146         | 38,418      |
| Consumer | Other consumer | Greater Than 90 Days                                                              |           |        |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                          |           |        |
| Total Past Due                                                                                                | $ 24,834       | $ 32,228    |

| X                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, Past Due, Percentage of Payment to Meet Contractual Requirement
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are 90 days or more past due and still accruing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5111-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are current.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953401-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953401-111524
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| 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
Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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