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Investments - Additional Information (Detail) (USD $)
9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Investment [Line Items]      
Securities pledged as collateral $ 2,600,000    
Gross realized gain on sale of securities 1,600,000 1,900,000  
Gross realized losses on sale of securities 0 22,000  
Recognized other-than-temporary impairment losses 0 193,000  
Fair value of investments reported at less than historical cost 11,294,000   28,458,000
Percentage of Bank portfolio 3.00%   9.00%
Pooled trust preferred securities, amortized cost 334,857,000   328,001,000
Pooled trust preferred securities, fair value 342,863,000   330,878,000
Pooled Trust Preferred Securities
     
Investment [Line Items]      
Gross realized losses on sale of securities       
Pooled trust preferred securities, amortized cost 6,743,000    
Pooled trust preferred securities, book value 8,000,000    
Pooled trust preferred securities, fair value $ 2,408,000    
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%