Annual report pursuant to Section 13 and 15(d)

Subordinated Debentures - Additional Information (Detail)

v3.8.0.1
Subordinated Debentures - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Sep. 30, 2014
Aug. 31, 2014
Debt Instrument [Line Items]              
Subordinated debentures     $ 117,270,000 $ 18,978,000      
Subordinated Debentures [Member]              
Debt Instrument [Line Items]              
Subordinated debentures, unpaid principal amount $ 100,000,000            
Debt instrument fixed rate (percent) 5.45%            
Debt issuance costs $ 2,200,000            
Long-term debt     98,000,000        
Amortization of debt issuance costs     134,000        
Central Bancorp, Inc [Member]              
Debt Instrument [Line Items]              
Debt instrument discount             $ 8,300,000
Amortization of subordinated debentures     $ 329,000 275,000 $ 176,000    
Central Bancorp, Inc [Member] | Subordinated Debentures [Member]              
Debt Instrument [Line Items]              
Subordinated debentures, unpaid principal amount   $ 26,000,000       $ 26,800,000  
Subordinated debentures           18,500,000  
Debt instrument discount           $ 8,300,000  
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     $ 329,000 $ 275,000 $ 176,000    
3-Month LIBOR [Member] | Subordinated Debentures [Member]              
Debt Instrument [Line Items]              
Basis spread (percent) 3.315%            
3-Month LIBOR [Member] | Central Bancorp, Inc [Member] | Subordinated Debentures [Member]              
Debt Instrument [Line Items]              
Basis spread (percent)   1.40%