| **LOANS AND ALLOWANCE FOR CREDIT LOSSES (Portfolio Segments and Classes) (Details) - USD ($) $ in Thousands**                                                                                                                                                                                                                                                                                                                                                                                                   | Jun. 30, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | [1],[2]       | $ 80,570,776  | $ 79,373,792 |
| Commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 41,100,316    | 40,625,720 |
| Commercial | Corporate banking                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 2,885,532     | 2,949,089 |
| Commercial | Middle market commercial real estate                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 4,872,407     | 4,223,359 |
| Commercial | Santander real estate capital                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 1,494,804     | 1,550,469 |
| Commercial | Commercial real estate loans                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 9,252,743     | 8,722,917 |
| Commercial | Commercial and industrial loans                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 19,615,226    | 19,787,834 |
| Loans held-for-sale                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |               | 464,300       | 86,400 |
| Commercial | Multifamily loans                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 9,225,985     | 9,438,463 |
| Commercial | Other commercial                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 3,006,362     | 2,676,506 |
| Consumer                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 39,470,460    | 38,748,072 |
| Consumer | Residential mortgages                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-sale                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |               | 345,200       | 236,800 |
| Consumer | Retail installment contracts and auto loans                                                                                                                                                                                                                                                                                                                                                                                                                                                          |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 17,250        | 17,844 |
| Loans held-for-sale                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |               | 1,500,000     | 905,700 |
| Consumer | Personal unsecured loans                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-sale                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |               | 970,700       | 2,000,000 |
| Consumer loans secured by real estate                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 12,305,770    | 12,382,227 |
| Consumer loans secured by real estate | Residential mortgages                                                                                                                                                                                                                                                                                                                                                                                                                                                   |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 6,227,307     | 6,230,995 |
| Consumer loans secured by real estate | Home equity loans and lines of credit                                                                                                                                                                                                                                                                                                                                                                                                                                   |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 6,078,463     | 6,151,232 |
| Consumer loans not secured by real estate | Retail installment contracts and auto loans - originated                                                                                                                                                                                                                                                                                                                                                                                                            |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 20,903,940    | 18,539,588 |
| Consumer loans not secured by real estate | Retail installment contracts and auto loans - purchased                                                                                                                                                                                                                                                                                                                                                                                                             |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 4,629,815     | 6,108,210 |
| Consumer loans not secured by real estate | Personal unsecured loans                                                                                                                                                                                                                                                                                                                                                                                                                                            |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | 722,215       | 685,467 |
| Consumer loans not secured by real estate | Other consumer                                                                                                                                                                                                                                                                                                                                                                                                                                                      |               |          |   |
| **Loans Receivable [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |               |          |   |
| Loans held-for-investment                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |               | $ 908,720     | $ 1,032,580 |
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| [1]Loans held-for-investment ("LHFI") includes $250.7 million and $328.7 million of loans recorded at fair value at June 30, 2016 and December 31, 2015, respectively.[2]The Company has interests in certain securitization trusts ("Trusts") that are considered variable interest entities ("VIEs") for accounting purposes. The Company consolidates VIEs where it is deemed the primary beneficiary. See Note 6 to these Condensed Consolidated Financial Statements for additional information. |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.
+ References
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| - Definition
Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.
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