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Securities Available for Sale - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Investment
Dec. 31, 2009
Investment
Schedule of Available-for-sale Securities [Line Items]            
Available for sale securities pledged to secure deposits of public funds and trust deposits, carrying value $ 200,315   $ 200,315   $ 254,042  
Proceeds from sales of securities available for sale     272,900 136,532    
Gross realized gains on sale of securities available for sale     3,808 2,449    
Gross realized losses on sale of securities available for sale     725      
Impairment charge related to potential credit loss 0 50 0 50 200 400
Bond whose impairment was deemed         1 1
Estimated fair value 7,277   7,277   8,996  
Obligations of U.S. Government sponsored enterprises
           
Schedule of Available-for-sale Securities [Line Items]            
Percentages of mortgage-backed securities represented by U.S. Government-sponsored enterprise securities 99.10%   99.10%      
Estimated fair value 0   0   6,207  
Mortgage-backed securities
           
Schedule of Available-for-sale Securities [Line Items]            
Accumulated other comprehensive loss (pre-tax)     522   736  
Estimated fair value 7,277   7,277   2,789  
Mortgage-backed securities | Other than temporarily impaired securities
           
Schedule of Available-for-sale Securities [Line Items]            
Estimated fair value $ 2,774   $ 2,774      
Unrealized losses 94.10%   94.10%      
Continuous Unrealized Loss Position Less Than Twelve Months
           
Schedule of Available-for-sale Securities [Line Items]            
Investment securities that were in an unrealized loss position or impaired for time frame 16   16      
Continuous Unrealized Loss Position More Than Twelve Months
           
Schedule of Available-for-sale Securities [Line Items]            
Investment securities that were in an unrealized loss position or impaired for time frame 2   2