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
Aug. 20, 2021
Dec. 31, 2005
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Dec. 31, 2014
Debt Instrument [Line Items]            
Overnight advances from federal home loan banks     $ 60,000,000   $ 212,500,000  
Overnight advances (percent)     5.69%   5.70%  
Advances from the FHLB     $ 112,500,000   $ 112,500,000  
Weighted average interest rate of outstanding advances (percent)     3.91%   2.77%  
Interest expense on borrowings     $ 1,655,000 $ 2,369,000    
Pledged loans receivable, carrying value     2,450,000,000   $ 2,360,000,000  
Remaining available borrowing capacity     1,330,000,000   1,090,000,000.00  
Loans pledged with FRB     23,800,000   24,800,000  
Borrowing capacity through Fed Discount Window     22,300,000   23,200,000  
Subordinated debentures issued     136,800,000   136,800,000  
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     $ 108,400,000   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     $ 21,800,000   21,700,000  
Unpaid principal balance           $ 26,800,000
Estimated fair value           18,500,000
Debt instrument discount     $ 5,000,000   $ 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     $ 106,000,000 $ 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     6.99%      
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%