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Premises and Equipment - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Property, Plant and Equipment [Line Items]      
Amount paid under sale-leaseback arrangement $ 431,000    
Gain on sale of branch 297,000    
Sale-leaseback transaction 15 year    
Total lease expense 4,300,000 4,400,000 4,400,000
Premises and Equipment [Member]
     
Property, Plant and Equipment [Line Items]      
Depreciation included in noninterest expense $ 7,900,000 $ 8,300,000 $ 8,600,000