Quarterly report pursuant to Section 13 or 15(d)

Debt - Additional Information (Detail)

v3.5.0.2
Debt - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Dec. 31, 2005
Dec. 31, 2005
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Aug. 31, 2014
Debt Instrument [Line Items]                
FHLB advances     $ 55,000,000   $ 55,000,000   $ 170,000,000  
Interest on FHLB advances     $ 179,000 $ 1,000 $ 673,000 $ 61,000    
Weighted-average interest rate     0.47% 0.21% 0.44% 0.18%    
Loans pledged as collateral     $ 1,000,000,000   $ 1,000,000,000      
Borrowing capacity     751,300,000   751,300,000      
Available borrowing capacity     696,300,000   696,300,000      
Loans pledged with FRB     11,300,000   11,300,000      
Borrowing capacity through Fed Discount Window     11,100,000   11,100,000      
Outstanding borrowings     0   0   $ 0  
Amortization of subordinated debentures     $ 67,000 $ 46,000 $ 185,000 $ 125,000    
Central Bancorp, Inc [Member] | Subordinated Debentures [Member]                
Debt Instrument [Line Items]                
Unpaid principal balance               $ 26,800,000
Estimated fair value               18,500,000
Debt instrument discount               $ 8,300,000
Subordinated debentures issued $ 26,000,000.0 $ 26,000,000.0            
Fixed interest rate 6.26% 6.26%            
Term for the initial fixed interest rate   5 years            
Optional interest payment deferral period (not to exceed)   5 years            
Central Bancorp, Inc [Member] | Subordinated Debentures [Member] | 3-month London Interbank Offered Rate (LIBOR) [Member]                
Debt Instrument [Line Items]                
Basis spread 1.40%