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Securities Available for Sale - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]      
Maturity of contract Less than one year    
Approximate year of maturity 30 years    
Net securities gains $ 192,000 $ 2,185,000 $ 2,422,000
Decline in maturities 0 0  
Decline in sales 1,171,303,000 1,142,776,000  
Available for sale securities, unrealized loss position 28,960,000 32,486,000  
Available-for-sale securities pledged as collateral 631,000,000 668,800,000  
Non-bank subsidiaries [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Net securities gains   300,000  
Obligations of States and Political Subdivisions [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Investments in obligation   500,000 47,800,000
Decline in maturities   4,200,000  
Decline in sales   42,500,000  
Recognized gains   300,000  
Available for sale securities, unrealized loss position 82,000 400,000  
Single issuer trust preferred securities [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Sale of available securities   3,000,000  
Net securities gains   100,000  
Single issuer trust preferred securities [Member] | Non-bank subsidiaries [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Sale of available securities 5,100,000 5,200,000  
Pooled Trust Preferred Securities [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Net securities gains 200,000    
Pooled Trust Preferred Securities [Member] | Non-bank subsidiaries [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Sale of available securities 200,000    
Corporate Securities [Member] | Non-bank subsidiaries [Member]
      
Schedule of Available-for-sale Securities [Line Items]      
Sale of available securities   $ 1,200,000