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
Mar. 21, 2017
Dec. 31, 2005
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Dec. 31, 2014
Debt Instrument [Line Items]              
Overnight advances from federal home loan banks       $ 250,000,000.0   $ 250,000,000.0  
Overnight advances (percent)       5.11%   4.65%  
Advances from the FHLB       $ 100,000,000.0   $ 100,000,000.0  
Weighted average interest rate of outstanding advances (percent)       1.60%   0.87%  
Interest expense on borrowings       $ 2,369,000 $ 337,000    
Pledged loans receivable, carrying value       2,430,000,000   $ 1,990,000,000  
Remaining available borrowing capacity       1,150,000,000   1,070,000,000.00  
Loans pledged with FRB       22,900,000   23,400,000  
Borrowing capacity through Fed Discount Window       22,200,000   22,000,000.0  
Subordinated debentures issued       136,800,000   136,800,000  
Debt instrument, maturity date Sep. 01, 2031 Mar. 30, 2027          
2031 Notes              
Debt Instrument [Line Items]              
Subordinated debentures issued $ 110,000,000.0            
Fixed interest rate 3.75%            
Amortization of debt issuance costs $ 2,100,000            
Debt issuance costs, net       $ 108,200,000   108,200,000  
2027 Notes              
Debt Instrument [Line Items]              
Subordinated debentures issued   $ 100,000,000.0          
Fixed interest rate   5.45%          
Percentage of redemption price of outstanding principal amount.       100.00%      
Unamortized debt issuance cost       $ 1,100,000      
Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Subordinated debentures issued     $ 26,000,000.0        
Debt instrument, maturity date       Mar. 15, 2036      
Fixed interest rate     6.26%        
Debt outstanding, net of issuance cost       $ 21,300,000   21,200,000  
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount       $ 5,500,000   $ 5,600,000 $ 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       $ 104,000 $ 102,000    
Benchmark Rate | 2031 Notes              
Debt Instrument [Line Items]              
Basis spread 3.10%            
3-month London Interbank Offered Rate (LIBOR) | 2027 Notes              
Debt Instrument [Line Items]              
Basis spread   3.315%          
3-month London Interbank Offered Rate (LIBOR) | Subordinated debentures | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread     140.00%