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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]      
Goodwill $ 27,095,000 $ 27,095,000  
Change in carrying amount of goodwill 0 0  
Amortization of core deposit intangible assets 586,000 699,000 $ 711,000
Aggregate mortgage servicing rights amortization $ 72,000 $ 74,000 $ 29,000