| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Portfolio Segments and Classes) (Details) - USD ($)**     | Sep. 30, 2015    | Dec. 31, 2014    |
|----------------------------------------------------------------------------------------------------|------------------|------------------|
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | $ 39,824,202,000 | $ 36,640,761,000 |
| Corporate Banking                                                                                  |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 2,967,780,000    | 3,218,151,000    |
| Middle Markets Real Estate                                                                         |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 3,965,173,000    | 3,743,100,000    |
| Santander Real Estate Capital                                                                      |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 1,562,229,000    | 1,777,982,000    |
| Consumer loans secured by real estate                                                              |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 12,563,382,000   | 12,980,555,000   |
| Consumer loans secured by real estate | Residential mortgages                                      |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 6,408,164,000    | 6,773,575,000    |
| Consumer loans secured by real estate | Home equity loans and lines of credit                      |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 6,155,218,000    | 6,206,980,000    |
| Consumer loans not secured by real estate | Retail installment contracts and auto loans            |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 24,597,860,000   | 22,430,241,000   |
| Consumer loans not secured by real estate | Personal unsecured loans                               |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 644,548,000      | 2,696,820,000    |
| Consumer loans not secured by real estate | Other consumer                                         |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 1,079,729,000    | 1,303,279,000    |
| Commercial                                                                                         |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 39,824,202,000   | 36,621,667,000   |
| Commercial | Commercial real estate                                                                |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 8,495,182,000    | 8,739,233,000    |
| Commercial | Commercial and industrial loans                                                       |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 19,361,751,000   | 17,092,312,000   |
| Loans held-for-sale                                                                                | 0                | 19,100,000       |
| Commercial | Multi-family loans                                                                    |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 9,600,527,000    | 8,705,890,000    |
| Commercial | Other commercial                                                                      |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 2,366,742,000    | 2,084,232,000    |
| Commercial | Corporate Banking | Commercial real estate                                            |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 2,967,780,000    | 3,218,151,000    |
| Commercial | Middle Markets Real Estate | Commercial real estate                                   |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 3,965,173,000    | 3,743,100,000    |
| Commercial | Santander Real Estate Capital | Commercial real estate                                |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Total commercial loans                                                                             | 1,562,229,000    | 1,777,982,000    |
| Consumer                                                                                           |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 38,885,519,000   | 39,410,895,000   |
| Consumer | Residential mortgages                                                                   |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Loans held-for-sale                                                                                | 280,800,000      | 195,700,000      |
| Consumer | Retail installment contracts and auto loans                                             |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Loans held-for-sale                                                                                | 825,800,000      | 45,400,000       |
| Consumer | Personal unsecured loans                                                                |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Loans held-for-sale                                                                                | 1,900,000,000    | 0                |
| Consumer | Consumer loans secured by real estate                                                   |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 12,563,382,000   | 12,980,555,000   |
| Consumer | Consumer loans secured by real estate | Residential mortgages                           |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 6,408,164,000    | 6,773,575,000    |
| Consumer | Consumer loans secured by real estate | Home equity loans and lines of credit           |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 6,155,218,000    | 6,206,980,000    |
| Consumer | Consumer loans not secured by real estate | Retail installment contracts and auto loans |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 24,597,860,000   | 22,430,241,000   |
| Consumer | Consumer loans not secured by real estate | Personal unsecured loans                    |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | 644,548,000      | 2,696,820,000    |
| Consumer | Consumer loans not secured by real estate | Other consumer                              |             |             |
| **Loans Receivable [Line Items]**                                                                  |             |             |
| Consumer loans held for investment                                                                 | $ 1,079,729,000  | $ 1,303,279,000  |

| 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.
+ Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the net carrying amount of commercial loans and leases held in portfolio, after deducting deferred income, but before deducting the allowance for losses on commercial loans and leases.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524
+ Details**Name:**us-gaap_LoansAndLeasesReceivableCommercialNetOfDeferredIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the net carrying amount of consumer loans and leases held in portfolio, after deducting deferred income, but before deducting the allowance for losses on consumer loans and leases.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5074-111524
+ Details**Name:**us-gaap_LoansAndLeasesReceivableConsumerNetOfDeferredIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.
+ References
No definition available.
+ Details**Name:**us-gaap_LoansHeldForSaleFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
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