| **BORROWINGS (SHUSA) (Details) - USD ($) $ in Thousands**                                                                                                                                                                                                                                                                       | Sep. 30, 2015 | Dec. 31, 2014 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Total borrowings and other debt obligations                                                                                                                                                                                                                                                                                     | [1]           | $ 48,371,320  | $ 39,679,382 |
| SHUSA                                                                                                                                                                                                                                                                                                                           |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Total borrowings and other debt obligations                                                                                                                                                                                                                                                                                     |               | $ 3,292,911   | $ 1,804,297 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 3.91%         | 3.89% |
| SHUSA | 3.00% senior notes, due September 2015                                                                                                                                                                                                                                                                                  |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           |               | $ 0           | $ 598,788 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 0.00%         | 3.28% |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |               | 3.00%         |   |
| SHUSA | 4.625% senior notes, due April 2016                                                                                                                                                                                                                                                                                     |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           |               | $ 475,467     | $ 474,718 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 4.85%         | 4.85% |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |               | 4.625%        |   |
| SHUSA | 3.45% senior notes, due August 2018                                                                                                                                                                                                                                                                                     |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           |               | $ 497,604     | $ 497,025 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 3.62%         | 3.62% |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |               | 3.45%         |   |
| SHUSA | 2.65% senior notes, due April 2020                                                                                                                                                                                                                                                                                      |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           |               | $ 992,211     | $ 0 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 2.83%         | 0.00% |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |               | 2.65%         |   |
| SHUSA | 4.50% senior notes, due July 2025                                                                                                                                                                                                                                                                                       |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Senior notes, Balance                                                                                                                                                                                                                                                                                                           |               | $ 1,093,824   | $ 0 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 4.57%         | 0.00% |
| Stated Rate                                                                                                                                                                                                                                                                                                                     |               | 4.50%         |   |
| SHUSA | Junior subordinated debentures - Capital Trust VI, due June 2036                                                                                                                                                                                                                                                        |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Subordinated debentures, Balance                                                                                                                                                                                                                                                                                                |               | $ 69,768      | $ 69,751 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 7.91%         | 7.91% |
| SHUSA | Common securities - Capital Trust VI                                                                                                                                                                                                                                                                                    |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Common securities, Balance                                                                                                                                                                                                                                                                                                      |               | $ 10,000      | $ 10,000 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 7.91%         | 7.91% |
| SHUSA | Junior subordinated debentures - Capital Trust IX, due July 2036                                                                                                                                                                                                                                                        |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Subordinated debentures, Balance                                                                                                                                                                                                                                                                                                |               | $ 149,397     | $ 149,375 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 2.09%         | 2.04% |
| SHUSA | Common securities - Capital Trust IX                                                                                                                                                                                                                                                                                    |               |          |   |
| **Debt Instrument [Line Items]**                                                                                                                                                                                                                                                                                                |               |          |   |
| Common securities, Balance                                                                                                                                                                                                                                                                                                      |               | $ 4,640       | $ 4,640 |
| Effective Rate                                                                                                                                                                                                                                                                                                                  |               | 2.09%         | 2.04% |
|                                                                                                                                                                                                                                                                                                                                 |
| [1]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 7 to these Condensed Consolidated Financial Statements for additional information. |

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| - Definition
Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.
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| - Definition
Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.
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Contractual interest rate for funds borrowed, under the debt agreement.
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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.
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| - Definition
Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.
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