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 9 Months Ended
Aug. 20, 2021
Mar. 21, 2017
Dec. 31, 2005
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 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       $ 137,500,000   $ 137,500,000   $ 150,000,000.0  
Weighted average interest rate of outstanding advances (percent)       1.05%   1.05%   1.40%  
Interest expense on borrowings       $ 409,000 $ 582,000 $ 1,332,000 $ 1,838,000    
Borrowing       0   0   $ 0  
Pledged loans receivable, carrying value       2,170,000,000   2,170,000,000   2,170,000,000  
Remaining available borrowing capacity       1,370,000,000   1,370,000,000      
FHLB statutory lending limit       1,610,000,000   1,610,000,000   1,440,000,000  
Loans pledged with FRB       26,100,000   26,100,000   27,300,000  
Borrowing capacity through Fed Discount Window       24,600,000   24,600,000   26,300,000  
Subordinated debentures issued       224,100,000   224,100,000   126,800,000  
Debt instrument, maturity date Sep. 01, 2031 Mar. 30, 2027              
2017 Notes                  
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       86,100,000   86,100,000   98,500,000  
Subordinated notes acquired       12,700,000   12,700,000      
Debt instrument, premium       343,000,000,000   343,000,000,000      
Amortization of debt issuance costs           174,000      
Incremental interest expense           517,000      
2021 Notes                  
Debt Instrument [Line Items]                  
Subordinated debentures issued $ 110,000,000.0                
Fixed interest rate 3.75%                
Amortization of debt issuance costs $ 2,000,000.0                
Debt issuance costs, net       108,000,000.0   $ 108,000,000.0      
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       20,700,000   $ 20,700,000   20,400,000  
Unpaid principal balance                 $ 26,800,000
Estimated fair value                 18,500,000
Debt instrument discount       6,100,000   $ 6,100,000   $ 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       $ 102,000 $ 99,000 $ 300,000 $ 291,000    
3-month London Interbank Offered Rate (LIBOR) | 2017 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%            
Benchmark Rate | 2021 Notes                  
Debt Instrument [Line Items]                  
Basis spread 3.10%