Quarterly report [Sections 13 or 15(d)]

Borrowings and Subordinated Debentures - Additional Information (Detail)

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Borrowings and Subordinated Debentures - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended
Aug. 20, 2021
Dec. 31, 2005
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Sep. 30, 2024
Dec. 31, 2024
Dec. 31, 2014
Debt Instrument [Line Items]                  
Overnight advances from federal home loan banks     $ 90,000,000   $ 90,000,000     $ 225,000,000  
Overnight advances (percent)     4.64%   4.64%     4.78%  
Advances from the FHLB     $ 37,500,000   $ 37,500,000     $ 37,500,000  
Weighted average interest rate of outstanding advances (percent)     4.58%   4.58%     4.58%  
Interest expense on borrowings     $ 684,000 $ 1,896,000 $ 2,708,000 $ 3,551,000      
Pledged loans receivable, carrying value     2,400,000,000   2,400,000,000     $ 2,460,000,000  
Remaining available borrowing capacity     1,520,000,000   1,520,000,000     1,690,000,000  
Loans pledged with FRB     27,900,000   27,900,000     29,400,000  
Borrowing capacity through Fed Discount Window     26,100,000   26,100,000     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   108,600,000     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     22,300,000   22,300,000     22,100,000  
Unpaid principal balance                 $ 26,800,000
Estimated fair value                 18,500,000
Debt instrument discount     4,500,000   $ 4,500,000     $ 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 $ 104,000,000          
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     5.98%   5.98%        
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%