Quarterly report pursuant to Section 13 or 15(d)

Acquisition - Additional Information (Detail)

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Acquisition - Additional Information (Detail) (USD $)
 0 Months Ended 3 Months Ended 9 Months Ended
Aug. 31, 2014
Sep. 30, 2014
Sep. 30, 2014
Branches
Sep. 30, 2013
Aug. 31, 2014
Loans At Acquisition Date [Line Items]          
Number of branch locations     49    
Bargain purchase gain on acquisition   $ 6,593,000 $ 6,593,000    
Merger and integration expense   3,415,000 3,572,000    
Core deposit intangible         2,213,000
Core deposit intangible, useful life 10 years        
Amortization expense of core deposit intangible   33,000 33,000 123,000  
Certificates of deposit         11,300,000
Amortization of certificates of deposit   591,000      
Discount on subordinated debentures         8,300,000
Amortization of subordinated debentures   35,000 36,000    
Central Bancorp, Inc [Member]
          
Loans At Acquisition Date [Line Items]          
Consideration paid for redemption of preferred stock and cumulative unpaid dividends 28,675,000        
Common stock, price per share after acquisition         $ 17.64
Consideration paid 50,000,000        
Total assets acquired         1,255,929,000
Total liabilities assumed         1,169,095,000
Total identifiable net assets         86,834,000
Loans         294,032,000
Total deposits         1,098,997,000
Number of branch locations     23    
Bargain purchase gain on acquisition 6,593,000        
Merger and integration expense     3,572,000    
Core deposit intangible         2,213,000
Subordinated Debentures [Member] | Central Bancorp, Inc [Member]
          
Loans At Acquisition Date [Line Items]          
Consideration paid for accrued interest on subordinated debentures 1,566,000        
Discount on subordinated debentures   8,300,000 8,300,000    
Amortization of subordinated debentures   $ 35,000