Annual report [Section 13 and 15(d), not S-K Item 405]

Subordinated Debentures - Additional Information (Detail)

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Subordinated Debentures - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Aug. 20, 2021
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2014
Debt Instrument [Line Items]              
Subordinated debentures issued       $ 136,800,000 $ 136,800,000    
Debt instrument, maturity date Sep. 01, 2031 Mar. 30, 2027          
Amortization of debt issuance costs $ 2,100,000            
2031 Notes              
Debt Instrument [Line Items]              
Subordinated debentures issued $ 110,000,000.0            
Fixed interest rate 3.75%            
Amortization of debt issuance costs       200,000 200,000 $ 200,000  
Debt issuance costs, net       $ 108,500,000 108,300,000    
2031 Notes | Benchmark Rate              
Debt Instrument [Line Items]              
Basis spread 3.10%            
2017 Notes              
Debt Instrument [Line Items]              
Subordinated debentures issued   $ 100,000,000.0          
Fixed interest rate   5.45%          
Debt issuance cost   $ 2,300,000          
2017 Notes | 3-month London Interbank Offered Rate (LIBOR)              
Debt Instrument [Line Items]              
Basis spread   3.32%          
Subordinated debentures              
Debt Instrument [Line Items]              
Amortization of debt issuance costs           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% 6.02%      
Debt outstanding, net of issuance cost       $ 22,100,000 21,700,000    
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount       $ 4,700,000 5,100,000   $ 8,300,000
Frequency of interest payment       quarterly      
Term for the initial fixed interest rate       5 years      
Variable interest rate basis       three-month LIBOR plus 140 basis points      
Amortization of subordinated debentures       $ 400,000 $ 400,000 $ 400,000  
Optional interest payment deferral period (not to exceed)       5 years      
Subordinated debentures | 3-month London Interbank Offered Rate (LIBOR) | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread     140.00%        
Subordinated debentures | Three Month Secured Overnight Financing Rate SOFR | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread       166.00%      
2027 Notes              
Debt Instrument [Line Items]              
Percentage of redemption price of outstanding principal amount.           100.00%  
Unamortized debt issuance cost           $ 1,100,000  
Credit Spread | Subordinated debentures | Three Month Secured Overnight Financing Rate SOFR | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread       140.00%      
3-Month Secured Overnight Financing Rate (SOFR ) | Subordinated debentures | Three Month Secured Overnight Financing Rate SOFR | Central Bancorp, Inc              
Debt Instrument [Line Items]              
Basis spread       26.00%