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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