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 6 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Dec. 31, 2014
Debt Instrument [Line Items]                
Overnight advances from federal home loan banks     $ 0   $ 0   $ 0  
Advances from the FHLB     $ 150,000,000.0   $ 150,000,000.0   $ 150,000,000.0  
Weighted average interest rate of outstanding advances (percent)     1.20%   1.20%   1.40%  
Interest expense on borrowings     $ 447,000 $ 760,000 $ 933,000 $ 1,256,000    
Borrowing     0   0   $ 0  
Pledged loans receivable, carrying value     2,230,000,000   2,230,000,000   2,170,000,000  
Remaining available borrowing capacity     1,370,000,000   1,370,000,000      
FHLB statutory lending limit     1,650,000,000   1,650,000,000   1,440,000,000  
Loans pledged with FRB     40,500,000   40,500,000   27,300,000  
Borrowing capacity through Fed Discount Window     38,500,000   38,500,000   26,300,000  
Subordinated debentures issued     126,800,000   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   $ 98,600,000   98,500,000  
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     20,600,000   $ 20,600,000   20,400,000  
Unpaid principal balance               $ 26,800,000
Estimated fair value               18,500,000
Debt instrument discount     6,200,000   $ 6,200,000   $ 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 $ 198,000 $ 192,000    
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%