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INVESTMENTS (Narrative) (Detail) - USD ($) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2024 | Mar. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2023
Schedule of Investments [Line Items] |  |  |  |
Trading securities | $ 4,847,000 |  |  | $ 4,647,000
AFS debt securities, allowance | 0 |  |  | 0
Net unrealized loss on debt securities, net of tax | 65,700,000 |  |  | 60,500,000
Deferred tax assets, unrealized losses on available for sale securities | 18,000,000 |  |  | 16,600,000
Provision for credit losses, HTM debt securities | (910,000) | $ 1,838,000 |  |
Write off, accrued interest, HTM debt securities | 910,000 | $ (1,838,000) |  |
HTM debt securities, allowance | 0 |  |  | 0
HTM debt securities, excluding accrued interest from amortized cost | 1,500,000 |  |  | 1,700,000
Asset Pledged as Collateral |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
AFS and HTM debt securities, amortized cost | 736,800,000 |  |  | 675,500,000
AFS and HTM debt securities, fair value | 647,000,000 |  |  | 607,900,000
Available-for-sale Securities |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Interest receivable | 1,700,000 |  |  | 1,700,000
Transferred Securities |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
AFS debt securities transferred to HTM, fair value |  |  | $ 520,300,000 |
AFS debt securities, transfer, unrealized loss |  |  | $ 72,100,000 |
Net unrealized loss on debt securities, net of tax | (45,700,000) |  |  | (46,900,000)
Deferred tax assets, unrealized losses on available for sale securities | 12,500,000 |  |  | $ 12,800,000
Signature Bank Corporate Bond |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Write off, accrued interest, HTM debt securities | $ 8,000 |  |  |

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