Quarterly report pursuant to Section 13 or 15(d)

Debt - Additional Information (Detail)

v3.7.0.1
Debt - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2005
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Aug. 31, 2014
Debt Instrument [Line Items]              
FHLB advances       $ 50,000,000   $ 315,000,000  
Interest on FHLB advances       $ 468,000 $ 195,000    
Weighted-average interest rate       0.70% 0.43%    
Loans pledged as collateral       $ 1,000,000,000      
Borrowing capacity       760,300,000      
Available borrowing capacity       710,300,000      
Loans pledged with FRB       10,200,000      
Borrowing capacity through Fed Discount Window       9,900,000      
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 $ 2,000,000            
Debt outstanding, net of issuance cost       97,700,000      
Amortization of debt issue costs       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       19,100,000   19,000,000  
Unpaid principal balance             $ 26,800,000
Estimated fair value             18,500,000
Debt instrument discount       7,700,000   $ 7,800,000 $ 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       $ 77,000 $ 56,000    
Central Bancorp, Inc [Member] | Subordinated Debentures [Member] | 3-month London Interbank Offered Rate (LIBOR) [Member]              
Debt Instrument [Line Items]              
Basis spread   1.40%