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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
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 33,000 $ 167,000  
Aggregate mortgage servicing rights amortization $ 16,000 $ 11,000