Quarterly report pursuant to Section 13 or 15(d)

Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail)

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Subordinated Debentures and Rescinded Stock Obligation - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Dec. 31, 2005
Dec. 31, 2005
Sep. 30, 2015
Sep. 30, 2015
Dec. 31, 2014
Sep. 30, 2014
Aug. 31, 2014
Debt Instrument [Line Items]              
Amortization of subordinated debentures     $ 46,000 $ 125,000      
Rescinded stock obligation     0 0 $ 933,000   $ 15,500,000
Accrued interest payable         $ 288,000   4,500,000
Central Bancorp, Inc [Member]              
Debt Instrument [Line Items]              
Estimated fair value             18,473,000
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 $ 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          
Variable interest rate basis   3 month LIBOR plus 140 basis          
Optional interest payment deferral period (not to exceed)   5 years          
Amortization of subordinated debentures     $ 46,000 $ 125,000      
3-month London Interbank Offered Rate (LIBOR) [Member] | Central Bancorp, Inc [Member] | Subordinated Debentures [Member]              
Debt Instrument [Line Items]              
Basis spread 1.40%