Quarterly report pursuant to Section 13 or 15(d)

Debt - Additional Information (Detail)

v3.4.0.3
Debt - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended
Dec. 31, 2005
Dec. 31, 2005
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Debt Instrument [Line Items]          
FHLB advances     $ 250,000,000   $ 170,000,000
Interest on FHLB advances     $ 195,000 $ 56,000  
Weighted-average interest rate     0.43% 0.18%  
Loans pledged as collateral     $ 472,700,000    
Borrowing capacity     422,500,000    
Available borrowing capacity     172,500,000    
Loans pledged with FRB     46,200,000    
Borrowing capacity through Fed Discount Window     29,600,000    
Outstanding borrowings     0   $ 0
Amortization of subordinated debentures     $ 56,000 $ 38,000  
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%