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
Mar. 21, 2017
Dec. 31, 2005
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2014
Debt Instrument [Line Items]            
Overnight advances from federal home loan banks     $ 0   $ 0  
Advances from the FHLB     $ 150,000,000.0   $ 150,000,000.0  
Weighted average interest rate of outstanding advances (percent)     1.20%   1.40%  
Interest expense on borrowings     $ 478,000 $ 496,000    
Borrowing     0   $ 0  
Pledged loans receivable, carrying value     2,340,000,000   2,170,000,000  
Remaining available borrowing capacity     1,390,000,000      
FHLB statutory lending limit     1,570,000,000   1,440,000,000  
Loans pledged with FRB     32,600,000   27,300,000  
Borrowing capacity through Fed Discount Window     31,100,000   26,300,000  
Subordinated debentures issued     126,800,000   126,800,000  
Debt instrument, maturity date Mar. 30, 2027          
Subordinated debentures            
Debt Instrument [Line Items]            
Subordinated debentures issued $ 100,000,000.0          
Fixed interest rate 5.45%          
Debt issuance costs $ 2,300,000          
Debt outstanding, net of issuance cost     98,600,000   98,500,000  
Amortization of debt issuance costs     $ 53,000 50,000    
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     $ 20,500,000   20,400,000  
Unpaid principal balance           $ 26,800,000
Estimated fair value           18,500,000
Debt instrument discount     $ 6,300,000   $ 6,400,000 $ 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     $ 99,000 $ 96,000    
3-month London Interbank Offered Rate (LIBOR) | Subordinated debentures            
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%