| **BUSINESS SEGMENT INFORMATION - Segment Information (Details) - USD ($) $ in Thousands** | 12 Months Ended |
|-------------------------------------------------------------------------------------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | $ 5,875,348     | $ 6,174,463 | $ 6,189,521 |
| Non-interest income                                                                       | 3,464,849       | 3,729,263 | 4,452,187 |
| Credit loss expense / (benefit)                                                           | 2,226,438       | 2,018,817 | (207,349) |
| Total expenses                                                                            | 6,327,003       | 6,136,779 | 6,144,167 |
| Income/(loss) before income taxes                                                         | 786,756         | 1,748,130 | 4,704,890 |
| Total assets                                                                              | 164,972,575     | 168,194,320 | 159,821,231 |
| Reportable Segments | Auto                                                                |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 3,656,625       | 4,041,002 | 4,335,818 |
| Non-interest income                                                                       | 2,480,455       | 2,778,835 | 3,412,277 |
| Credit loss expense / (benefit)                                                           | 1,798,268       | 1,730,062 | (118,671) |
| Total expenses                                                                            | 3,271,352       | 3,291,610 | 3,454,854 |
| Income/(loss) before income taxes                                                         | 1,067,460       | 1,798,165 | 4,411,912 |
| Total assets                                                                              | 61,712,245      | 62,645,083 | 63,061,949 |
| Reportable Segments | CBB                                                                 |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 1,611,634       | 1,403,930 | 1,300,240 |
| Non-interest income                                                                       | 259,679         | 300,316 | 324,260 |
| Credit loss expense / (benefit)                                                           | 301,884         | 216,600 | 29,339 |
| Total expenses                                                                            | 1,460,244       | 1,523,939 | 1,564,410 |
| Income/(loss) before income taxes                                                         | 109,185         | (36,293) | 30,751 |
| Total assets                                                                              | 12,085,567      | 13,107,982 | 12,561,672 |
| Reportable Segments | C&I                                                             |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 325,667         | 311,808 | 291,550 |
| Non-interest income                                                                       | 55,579          | 63,124 | 63,502 |
| Credit loss expense / (benefit)                                                           | (23,299)        | 47,820 | (78,402) |
| Total expenses                                                                            | 256,858         | 258,014 | 254,925 |
| Income/(loss) before income taxes                                                         | 147,687         | 69,098 | 178,529 |
| Total assets                                                                              | 4,994,784       | 6,345,307 | 6,915,480 |
| Reportable Segments | CRE                                                                 |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 444,467         | 357,676 | 341,362 |
| Non-interest income                                                                       | 24,774          | 36,396 | 40,932 |
| Credit loss expense / (benefit)                                                           | 176,183         | 7,321 | (7,186) |
| Total expenses                                                                            | 133,731         | 132,454 | 124,366 |
| Income/(loss) before income taxes                                                         | 159,327         | 254,297 | 265,114 |
| Total assets                                                                              | 23,787,283      | 20,546,103 | 17,379,967 |
| Reportable Segments | CIB                                                                 |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 201,501         | 178,435 | 107,568 |
| Non-interest income                                                                       | 365,340         | 245,967 | 255,337 |
| Credit loss expense / (benefit)                                                           | (20,666)        | 19,652 | (27,641) |
| Total expenses                                                                            | 622,808         | 459,153 | 286,982 |
| Income/(loss) before income taxes                                                         | (35,301)        | (54,403) | 103,564 |
| Total assets                                                                              | 26,416,104      | 30,478,602 | 16,016,527 |
| Reportable Segments | Wealth Management                                                   |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | 273,249         | 177,627 | 95,014 |
| Non-interest income                                                                       | 248,669         | 258,939 | 255,200 |
| Credit loss expense / (benefit)                                                           | 205             | 0 | (351) |
| Total expenses                                                                            | 279,425         | 244,862 | 223,831 |
| Income/(loss) before income taxes                                                         | 242,288         | 191,704 | 126,734 |
| Total assets                                                                              | 7,576,807       | 7,854,953 | 8,051,489 |
| Reportable Segments | Other                                                               |            |   |   |
| **Segment Reporting Information [Line Items]**                                            |            |   |   |
| Net interest income / (expense)                                                           | (637,795)       | (296,015) | (282,031) |
| Non-interest income                                                                       | 30,353          | 45,686 | 100,679 |
| Credit loss expense / (benefit)                                                           | (6,137)         | (2,638) | (4,437) |
| Total expenses                                                                            | 302,585         | 226,747 | 234,799 |
| Income/(loss) before income taxes                                                         | (903,890)       | (474,438) | (411,714) |
| Total assets                                                                              | $ 28,399,785    | $ 27,216,290 | $ 35,834,147 |

| X                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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| - Definition
Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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| - Definition
The aggregate total of expenses of managing and administering the affairs of an entity, including affiliates of the reporting entity, which are not directly or indirectly associated with the manufacture, sale or creation of a product or product line.
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| - Definition
Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.
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The net amount of operating interest income (expense).
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The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.
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