Quarterly report pursuant to Section 13 or 15(d)

Borrowings and Subordinated Debentures - Additional Information (Detail)

v3.19.3
Borrowings and Subordinated Debentures - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 21, 2017
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2005
Dec. 31, 2018
Dec. 31, 2014
Debt Instrument [Line Items]                
Borrowings   $ 75,000,000   $ 75,000,000     $ 55,000,000  
Weighted average interest rate of outstanding advances (percent)   1.71%   1.71%     1.94%  
Overnight advances   $ 0   $ 0     $ 55,000,000  
Interest on borrowings   367,000 $ 1,264,000 439,000 $ 2,959,000      
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   769,600,000   769,600,000        
Loans pledged with FRB   30,200,000   30,200,000        
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 $ 100,000,000.0              
Fixed interest rate 5.45%              
Debt issue cost $ 2,300,000              
Debt outstanding, net of issuance cost   98,300,000   98,300,000     98,100,000  
Amortization of debt issue costs   49,000 46,000 144,000 136,000      
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    
Fixed interest rate           6.26%    
Debt outstanding, net of issuance cost   19,900,000   19,900,000     19,700,000  
Unpaid principal balance   26,800,000   26,800,000        
Estimated fair value               $ 18,500,000
Debt instrument discount   6,900,000   6,900,000     $ 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   $ 96,000 $ 92,000 $ 280,000 $ 264,000      
Central Bancorp, Inc | Subordinated Debentures | 3-month London Interbank Offered Rate (LIBOR)                
Debt Instrument [Line Items]                
Basis spread           1.40%