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Premises and Equipment - Additional Information (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]        
Impairment loss     $ 2,600,000  
Fair value, net of selling costs, of held-for-sale asset     3,400,000  
Gain on eminent domain condemnation   $ 2,894,000 0 $ 0
Gain on sale of property plant equipment $ 542,000      
Premises and equipment, net   46,166,000 42,086,000  
Rental expense   5,900,000 $ 5,600,000 $ 5,500,000
Building [Member]        
Property, Plant and Equipment [Line Items]        
Premises and equipment, net   $ 450,000