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
Dec. 31, 2005
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2014
Debt Instrument [Line Items]                  
Borrowings       $ 0   $ 0   $ 55,000,000  
Interest on borrowings       0 $ 1,015,000 $ 72,000 $ 1,694,000    
Weighted-average interest rate         1.90% 2.74% 1.74%    
Loans pledged as collateral       1,200,000,000   $ 1,200,000,000      
Borrowing capacity       1,000,000,000.0   1,000,000,000.0      
Available borrowing capacity       905,000,000.0   905,000,000.0      
Loans pledged with FRB       30,000,000.0   30,000,000.0      
Borrowing capacity through Fed Discount Window       29,600,000   29,600,000      
Outstanding borrowings       0   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   98,200,000   98,100,000  
Amortization of debt issue costs       48,000 $ 45,000 95,000 $ 90,000    
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       19,900,000   19,900,000   19,700,000  
Unpaid principal balance                 $ 26,800,000
Estimated fair value                 18,500,000
Debt instrument discount       7,000,000.0   7,000,000.0   $ 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 $ 184,000 $ 172,000    
Central Bancorp, Inc | Subordinated Debentures | 3-month London Interbank Offered Rate (LIBOR)                  
Debt Instrument [Line Items]                  
Basis spread   1.40%