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
Aug. 20, 2021
Mar. 21, 2017
Dec. 31, 2005
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Dec. 31, 2021
Dec. 31, 2014
Debt Instrument [Line Items]                  
Overnight advances from federal home loan banks       $ 20,000,000.0   $ 20,000,000.0   $ 0  
Overnight advances (percent)       1.67%   1.67%      
Advances from the FHLB       $ 125,000,000.0   $ 125,000,000.0   $ 137,500,000  
Weighted average interest rate of outstanding advances (percent)       1.04%   1.04%   1.05%  
Interest expense on borrowings       $ 370,000 $ 447,000 $ 707,000 $ 933,000    
Pledged loans receivable, carrying value       2,450,000,000   2,450,000,000   $ 2,300,000,000  
Remaining available borrowing capacity       1,470,000,000   1,470,000,000      
FHLB statutory lending limit       1,780,000,000   1,780,000,000   1,610,000,000  
Loans pledged with FRB       27,300,000   27,300,000   34,700,000  
Borrowing capacity through Fed Discount Window       25,400,000   25,400,000   32,800,000  
Subordinated debentures issued       136,800,000   136,800,000   224,100,000  
Debt instrument, maturity date Sep. 01, 2031 Mar. 30, 2027              
2021 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       $ 108,100,000   $ 108,100,000   108,000,000.0  
2017 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%   100.00%      
Unamortized debt issuance cost       $ 1,100,000   $ 1,100,000      
Debt outstanding, net of issuance cost       0   $ 0   86,200,000  
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       21,000,000.0   $ 21,000,000.0   20,800,000  
Unpaid principal balance                 $ 26,800,000
Estimated fair value                 18,500,000
Debt instrument discount       5,800,000   $ 5,800,000   $ 6,000,000.0 $ 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 $ 204,000 $ 198,000    
Benchmark Rate | 2021 Notes                  
Debt Instrument [Line Items]                  
Basis spread 3.10%                
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%