Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Parenthetical) (Detail)

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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Parenthetical) (Detail) (Non-recurring Basis [Member], USD $)
Sep. 30, 2014
Dec. 31, 2013
Impaired Loans [Member] | Real estate loans [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Impaired loans $ 33,600,000 $ 32,200,000
Impaired Loans [Member] | Commercial and industrial loans [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Impaired loans 1,200,000 2,800,000
Impaired Loans [Member] | Consumer loans [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Impaired loans 1,800,000 1,300,000
Other Real Estate Owned [Member] | Real estate loans [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Non-performing loans held for sale 24,800,000 756,000
Other Real Estate Owned [Member] | Central Bancorp, Inc [Member] | Real estate loans [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Non-performing loans held for sale $ 22,300,000