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Investments - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Investment [Line Items]      
Securities pledged as collateral $ 2,800,000    
Gross realized gain on sale of securities 339,000 197,000  
Proceed from sale of securities 17,429,000 6,659,000  
Fair value of investments reported at less than historical cost 34,600,000   13,300,000
Percentage of Bank portfolio 12.00%   5.00%
Pooled trust preferred securities, amortized cost 280,689,000   276,326,000
Pooled trust preferred securities, fair value 284,271,000   281,197,000
Pooled Trust Preferred Securities
     
Investment [Line Items]      
Pooled trust preferred securities, amortized cost 6,744,000    
Pooled trust preferred securities, book value 8,000,000    
Pooled trust preferred securities, fair value $ 2,465,000    
Banks, thrifts or other depository institutions to all projected defaults
     
Investment [Line Items]      
Collateral recovery probability percentage 10.00%    
Insurance Companies to all projected insurance defaults
     
Investment [Line Items]      
Collateral recovery probability percentage 15.00%