| **INVESTMENT SECURITIES - Securities Pledged as Collateral (Details) - USD ($)** | 9 Months Ended  | 12 Months Ended |
|----------------------------------------------------------------------------------|-----------------|-----------------|
||
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**           |            |            |   |
| Securities pledged as collateral, fair value                                     | $ 5,300,000,000 |            | $ 5,300,000,000 |
| Accrued interest on investment securities                                        | 69,800,000      |            | 26,900,000 |
| Debt securities, available-for-sale, accrued interest writeoff                   | 0               |            | 0 |
| Debt securities, held-to-maturity, accrued interest, writeoff                    | 0               |            | 0 |
| AFS investment securities transferred to HTM investment securities               | 2,982,195,000   | $ 0             |   |
| Collateral with Federal Reserve Bank                                             |            |            |   |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**           |            |            |   |
| Securities pledged as collateral, fair value                                     | 2,000,000,000   |            | 1,600,000,000 |
| Public fund deposits                                                             |            |            |   |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**           |            |            |   |
| Securities pledged as collateral, fair value                                     | 2,900,000,000   |            | 3,200,000,000 |
| Repurchase agreements, hedging activities and recourse on loan sales             |            |            |   |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**           |            |            |   |
| Securities pledged as collateral, fair value                                     | 71,400,000      |            | 72,600,000 |
| Overnight customer deposits                                                      |            |            |   |
| **Financial Instruments Owned and Pledged as Collateral [Line Items]**           |            |            |   |
| Securities pledged as collateral, fair value                                     | $ 393,000,000   |            | $ 403,800,000 |

| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Securities, Available-For-Sale, Transferred To Held-To-Maturity Securities
+ References
No definition available.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of assets pledged to secure a debt instrument.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of writeoff of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized by reversing interest income.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 3D -URI https://asc.fasb.org/extlink&oid=124269663&loc=SL120267862-210455
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of writeoff of accrued interest on investment in debt security measured at amortized cost (held-to-maturity) recognized by reversing interest income.
+ References
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| 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.
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| X                                                                                                                                                                  |
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| X                                                                                                                                                    |
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| X                                                                                                                                                           |
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