Annual report pursuant to Section 13 and 15(d)

Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail)

v3.3.1.900
Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Dec. 31, 2005
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Sep. 30, 2014
Aug. 31, 2014
Debt Instrument [Line Items]            
Subordinated debentures   $ 18,703 $ 18,544      
Rescinded stock obligation   0 933     $ 15,500
Accrued interest payable           4,500
Rescinded stock obligation paid off   933 14,552 $ 0    
Interest payable   288        
Central Bancorp, Inc [Member]            
Debt Instrument [Line Items]            
Debt instrument discount           $ 8,300
Amortization of subordinated debentures   $ 176 71      
Central Bancorp, Inc [Member] | Subordinated Debentures [Member]            
Debt Instrument [Line Items]            
Subordinated debentures, unpaid principal amount $ 26,000       $ 26,800  
Subordinated debentures         18,500  
Debt instrument discount         $ 8,300  
Fixed interest rate 6.26%          
Effective interest rate period 5 years          
Variable interest rate basis 3 month LIBOR plus 140 basis          
Maturity date of subordinated debentures Dec. 31, 2035          
Frequency of interest payment   quarterly        
Interest deferral period (up to)   5 years        
Amortization of subordinated debentures   $ 176 $ 71      
3-Month LIBOR [Member] | Central Bancorp, Inc [Member] | Subordinated Debentures [Member]            
Debt Instrument [Line Items]            
Basis spread 1.40%