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Derivatives and Hedging Activities - Additional Information (Detail)
3 Months Ended 9 Months Ended
Jun. 30, 2024
USD ($)
Contract
Jun. 30, 2023
USD ($)
Jun. 30, 2024
USD ($)
Contract
Jun. 30, 2023
USD ($)
Dec. 31, 2023
USD ($)
Sep. 30, 2023
USD ($)
Derivative Instruments And Hedging Activities Disclosures [Line Items]            
Interest income (expense) $ 14,165,000 $ 15,244,000 $ 43,883,000 $ 46,014,000    
Derivative liability, collateral against obligations 0   0     $ 0
Designated as Hedging Instrument [Member] | Cash Flow Hedges of Interest Rate Risk [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]            
Derivative Instruments And Hedging Activities Disclosures [Line Items]            
Interest income (expense) 2,000,000 $ 2,500,000 6,700,000 $ 6,600,000    
Increase (decrease) in accrued interest payable         $ 4,900,000  
Designated as Hedging Instrument [Member] | Cash Flow Hedges of Interest Rate Risk [Member] | Interest Rate Swaps [Member] | Commercial Loans [Member] | Variable Rate [Member]            
Derivative Instruments And Hedging Activities Disclosures [Line Items]            
Derivative, notional principal amount 220,100,000   220,100,000      
Designated as Hedging Instrument [Member] | Cash Flow Hedges of Interest Rate Risk [Member] | Interest Rate Swaps [Member] | FHLB Advances [Member] | Variable Rate [Member]            
Derivative Instruments And Hedging Activities Disclosures [Line Items]            
Derivative, notional principal amount $ 280,000,000.0   $ 280,000,000.0      
Derivative, number of instruments | Contract 12   12