Quarterly report pursuant to Section 13 or 15(d)

Borrowings and Subordinated Debentures - Additional Information (Detail)

v3.8.0.1
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, 2018
Mar. 31, 2017
Dec. 31, 2017
Aug. 31, 2014
Debt Instrument [Line Items]              
Borrowings       $ 220,000,000   $ 150,000,000  
Interest on borrowings       $ 679,000 $ 468,000    
Weighted-average interest rate       1.54% 0.70%    
Loans pledged as collateral       $ 1,100,000,000      
Borrowing capacity       871,000,000      
Available borrowing capacity       651,000,000      
Loans pledged with FRB       8,100,000      
Borrowing capacity through Fed Discount Window       8,000,000      
Outstanding borrowings       0   0  
Subordinated Debentures [Member]              
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       98,000,000      
Amortization of debt issue costs       45,000 $ 4,000    
Subordinated Debentures [Member] | 3-month London Interbank Offered Rate (LIBOR) [Member]              
Debt Instrument [Line Items]              
Basis spread 3.315%            
Central Bancorp, Inc [Member] | Subordinated Debentures [Member]              
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       19,400,000   19,000,000  
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount       7,600,000   $ 7,700,000 $ 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       $ 86,000 $ 77,000    
Central Bancorp, Inc [Member] | Subordinated Debentures [Member] | 3-month London Interbank Offered Rate (LIBOR) [Member]              
Debt Instrument [Line Items]              
Basis spread   1.40%