Annual report pursuant to Section 13 and 15(d)

Subordinated Debentures - Additional Information (Detail)

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Subordinated Debentures - Additional Information (Detail) - Subordinated debentures - USD ($)
 1 Months Ended 12 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2014
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     $ 98,300,000 $ 98,100,000    
Amortization of debt issuance costs     193,000 182,000 $ 134,000  
Central Bancorp, Inc            
Debt Instrument [Line Items]            
Subordinated debentures issued   $ 26,000,000.0        
Fixed interest rate   6.26%        
Debt outstanding, net of issuance cost     20,000,000.0 19,700,000    
Unpaid principal balance           $ 26,800,000
Estimated fair value           18,500,000
Debt instrument discount     $ 6,800,000 7,100,000   $ 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     $ 376,000 $ 356,000 $ 329,000  
Debt instrument, maturity date     Mar. 15, 2036      
3-month London Interbank Offered Rate (LIBOR)            
Debt Instrument [Line Items]            
Basis spread 3.315%          
3-month London Interbank Offered Rate (LIBOR) | Central Bancorp, Inc            
Debt Instrument [Line Items]            
Basis spread   1.40%