Quarterly report pursuant to Section 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 9 Months Ended
Aug. 20, 2021
Dec. 31, 2005
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2014
Debt Instrument [Line Items]                
Overnight advances from federal home loan banks     $ 200,000,000   $ 200,000,000   $ 212,500,000  
Overnight advances (percent)     5.21%   5.21%   5.70%  
Advances from the FHLB     $ 100,000,000   $ 100,000,000   $ 112,500,000  
Weighted average interest rate of outstanding advances (percent)     4.02%   4.02%   2.77%  
Interest expense on borrowings     $ 1,561,000 $ 753,000 $ 5,112,000 $ 4,755,000    
Pledged loans receivable, carrying value     2,440,000,000   2,440,000,000   $ 2,360,000,000  
Remaining available borrowing capacity     1,240,000,000   1,240,000,000   1,090,000,000.00  
Loans pledged with FRB     31,500,000   31,500,000   24,800,000  
Borrowing capacity through Fed Discount Window     29,500,000   29,500,000   23,200,000  
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     108,500,000   $ 108,500,000   108,300,000  
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     22,000,000   $ 22,000,000   21,700,000  
Unpaid principal balance               $ 26,800,000
Estimated fair value               18,500,000
Debt instrument discount     4,800,000   $ 4,800,000   $ 5,100,000 $ 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 $ 324,000,000 $ 314,000,000    
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     6.61%   6.61%      
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%