Annual report pursuant to Section 13 and 15(d)

Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail)

v3.6.0.2
Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Aug. 31, 2014
Dec. 31, 2005
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Debt Instrument [Line Items]            
Subordinated debentures     $ 18,978 $ 18,703    
Rescinded stock obligation $ 15,500          
Accrued interest payable 4,500          
Rescinded stock obligation paid off 933   0 933 $ 14,552  
Interest payable 288          
Central Bancorp, Inc [Member]            
Debt Instrument [Line Items]            
Debt instrument discount $ 8,300          
Amortization of subordinated debentures     $ 275 176 $ 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 (percent)   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     $ 275 $ 176    
3-Month LIBOR [Member] | Central Bancorp, Inc [Member] | Subordinated Debentures [Member]            
Debt Instrument [Line Items]            
Basis spread (percent)   1.40%