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
Dec. 31, 2005
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Aug. 31, 2014
Debt Instrument [Line Items]              
Borrowings       $ 0   $ 55,000,000  
Interest on borrowings       $ 71,000 $ 679,000    
Weighted-average interest rate       2.71% 1.54%    
Loans pledged as collateral       $ 1,100,000,000      
Borrowing capacity       893,700,000      
Available borrowing capacity       753,700,000      
Loans pledged with FRB       30,000,000      
Borrowing capacity through Fed Discount Window       29,500,000      
Outstanding borrowings       0   0  
Subordinated Debentures              
Debt Instrument [Line Items]              
Subordinated debentures issued $ 100,000,000            
Fixed interest rate 5.45%            
Debt issue cost $ 2,300,000            
Debt outstanding, net of issuance cost       98,200,000   98,100,000  
Amortization of debt issue costs       47,000 $ 45,000    
Subordinated Debentures | 3-month London Interbank Offered Rate (LIBOR)              
Debt Instrument [Line Items]              
Basis spread 3.315%            
Central Bancorp, Inc | Subordinated Debentures              
Debt Instrument [Line Items]              
Subordinated debentures issued   $ 26,000,000.0 $ 26,000,000.0        
Fixed interest rate   6.26% 6.26%        
Debt outstanding, net of issuance cost       19,800,000   19,700,000  
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount       7,000,000   $ 7,100,000 $ 8,300,000
Term for the initial fixed interest rate     5 years        
Optional interest payment deferral period (not to exceed)     5 years        
Amortization of discount on debentures       $ 92,000 $ 86,000    
Central Bancorp, Inc | Subordinated Debentures | 3-month London Interbank Offered Rate (LIBOR)              
Debt Instrument [Line Items]              
Basis spread   1.40%