| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Portfolio Segments and Classes) (Details) - USD ($)** | Mar. 31, 2016    | Dec. 31, 2015    |
|------------------------------------------------------------------------------------------------|------------------|------------------|
| Commercial                                                                                     |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | $ 41,719,535,000 | $ 40,623,332,000 |
| Commercial | Corporate banking                                                                 |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 2,959,176,000    | 2,949,089,000    |
| Commercial | Middle market commercial real estate                                              |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 4,855,085,000    | 4,223,359,000    |
| Commercial | Santander real estate capital                                                     |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 1,501,030,000    | 1,550,469,000    |
| Commercial | Commercial real estate loans                                                      |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 9,315,291,000    | 8,722,917,000    |
| Commercial | Commercial and industrial loans                                                   |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 20,334,247,000   | 19,785,446,000   |
| Loans held-for-sale                                                                            | 0                | 86,400,000       |
| Commercial | Multifamily loans                                                                 |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 9,330,482,000    | 9,438,463,000    |
| Commercial | Other commercial                                                                  |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Commercial loans                                                                               | 2,739,515,000    | 2,676,506,000    |
| Consumer                                                                                       |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 39,494,072,000   | 38,859,478,000   |
| Consumer | Residential mortgages                                                               |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Loans held-for-sale                                                                            | 219,200,000      | 236,800,000      |
| Consumer | Retail installment contracts and auto loans                                         |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Loans held-for-sale                                                                            | 1,300,000,000    | 905,700,000      |
| Consumer | Personal unsecured loans                                                            |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Loans held-for-sale                                                                            | 978,800,000      | 2,000,000,000    |
| Consumer loans secured by real estate                                                          |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 12,323,811,000   | 12,377,908,000   |
| Consumer loans secured by real estate | Residential mortgages                                  |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 6,219,967,000    | 6,230,995,000    |
| Consumer loans secured by real estate | Home equity loans and lines of credit                  |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 6,103,844,000    | 6,146,913,000    |
| Consumer loans not secured by real estate | Retail installment contracts and auto loans        |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 25,503,907,000   | 24,763,523,000   |
| Consumer loans not secured by real estate | Personal unsecured loans                           |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | 693,328,000      | 685,467,000      |
| Consumer loans not secured by real estate | Other consumer                                     |             |             |
| **Loans Receivable [Line Items]**                                                              |             |             |
| Consumer loans held for investment                                                             | $ 973,026,000    | $ 1,032,580,000  |

| 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
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| 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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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                     |
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| X                                                                                                                                                                                                          |
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| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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