Quarterly report [Sections 13 or 15(d)]

Borrowings and Subordinated Debentures - Additional Information (Detail)

v3.25.1
Borrowings and Subordinated Debentures - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Aug. 20, 2021
Dec. 31, 2005
Mar. 31, 2025
Mar. 31, 2024
Sep. 30, 2024
Dec. 31, 2024
Dec. 31, 2014
Debt Instrument [Line Items]              
Overnight advances from federal home loan banks     $ 80,000,000     $ 225,000,000  
Overnight advances (percent)     4.65%     4.78%  
Advances from the FHLB     $ 37,500,000     $ 37,500,000  
Weighted average interest rate of outstanding advances (percent)     4.58%     4.58%  
Interest expense on borrowings     $ 2,024,000 $ 1,655,000      
Pledged loans receivable, carrying value     2,440,000,000     $ 2,460,000,000  
Remaining available borrowing capacity     1,430,000,000     1,690,000,000  
Loans pledged with FRB     28,900,000     29,400,000  
Borrowing capacity through Fed Discount Window     27,000,000     27,600,000  
Debt instrument, maturity date Sep. 01, 2031            
2031 Notes              
Debt Instrument [Line Items]              
Subordinated debentures issued $ 110,000,000            
Fixed interest rate 3.75%            
Amortization of debt issuance costs $ 2,100,000            
Debt issuance costs, net     108,600,000     108,500,000  
Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Subordinated debentures issued   $ 26,000,000          
Debt instrument, maturity date         Mar. 15, 2036    
Fixed interest rate   6.26%          
Debt outstanding, net of issuance cost     22,200,000     22,100,000  
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount     $ 4,600,000     $ 4,700,000 $ 8,300,000
Frequency of interest payment     quarterly        
Term for the initial fixed interest rate     5 years        
Optional interest payment deferral period (not to exceed)     5 years        
Variable interest rate basis     three-month LIBOR plus 140 basis points        
Amortization of subordinated debentures     $ 112,000,000 $ 106,000,000      
Benchmark Rate | 2031 Notes              
Debt Instrument [Line Items]              
Basis spread 3.10%            
3-month London Interbank Offered Rate (LIBOR) | Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Fixed interest rate     5.96%        
Basis spread   1.40%          
3-Month Secured Overnight Financing Rate (SOFR ) | Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread     1.66%        
3-Month Secured Overnight Financing Rate (SOFR ) | Credit Spread | Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread     1.40%        
3-Month Secured Overnight Financing Rate (SOFR ) | Adjustment to Convert 3-month LIBOR to 3-month SOFR | Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread     0.26%