Annual report pursuant to Section 13 and 15(d)

Subordinated Debentures - Additional Information (Detail)

v3.10.0.1
Subordinated Debentures - Additional Information (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Mar. 21, 2017
Dec. 31, 2005
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Debt Instrument [Line Items]            
Subordinated debentures     $ 117,808,000 $ 117,270,000    
Subordinated debentures            
Debt Instrument [Line Items]            
Subordinated debentures, unpaid principal amount $ 100,000,000          
Debt instrument fixed rate (percent) 5.45%          
Debt issuance costs $ 2,300,000          
Long-term debt     98,100,000 98,000,000    
Amortization of debt issuance costs     182,000 134,000    
Central Bancorp, Inc | Subordinated debentures            
Debt Instrument [Line Items]            
Subordinated debentures, unpaid principal amount   $ 26,000,000       $ 26,800,000
Debt instrument fixed rate (percent)   6.26%        
Long-term debt     19,700,000 19,300,000    
Subordinated debentures           18,500,000
Debt instrument discount     $ 7,100,000 7,500,000   $ 8,300,000
Variable interest rate basis   three-month LIBOR plus 140        
Frequency of interest payment     quarterly      
Interest deferral period (up to)     5 years      
Amortization of subordinated debentures     $ 356,000 $ 329,000 $ 275,000  
3-Month LIBOR | Subordinated debentures            
Debt Instrument [Line Items]            
Basis spread (percent) 3.315%          
3-Month LIBOR | Central Bancorp, Inc | Subordinated debentures            
Debt Instrument [Line Items]            
Basis spread (percent)   1.40%