Quarterly report pursuant to Section 13 or 15(d)

Debt - Additional Information (Detail)

v3.8.0.1
Debt - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2005
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Aug. 31, 2014
Debt Instrument [Line Items]                  
FHLB advances       $ 110,000,000   $ 110,000,000   $ 315,000,000  
Interest on FHLB advances       $ 198,000 $ 179,000 $ 714,000 $ 673,000    
Weighted-average interest rate       1.16% 0.47% 0.80% 0.44%    
Loans pledged as collateral       $ 1,100,000,000   $ 1,100,000,000      
Borrowing capacity       808,900,000   808,900,000      
Available borrowing capacity       698,900,000   698,900,000      
Loans pledged with FRB       9,300,000   9,300,000      
Borrowing capacity through Fed Discount Window       9,200,000   9,200,000      
Outstanding borrowings       0   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       97,900,000   97,900,000      
Amortization of debt issue costs       43,000   90,000      
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       19,200,000   19,200,000   19,000,000  
Unpaid principal balance                 $ 26,800,000
Estimated fair value                 18,500,000
Debt instrument discount       7,600,000   7,600,000   $ 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       $ 85,000 $ 67,000 $ 243,000 $ 185,000    
Central Bancorp, Inc [Member] | Subordinated Debentures [Member] | 3-month London Interbank Offered Rate (LIBOR) [Member]                  
Debt Instrument [Line Items]                  
Basis spread   1.40%