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Goodwill and Intangible Assets - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 14, 2018
Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]            
Goodwill $ 76,316   $ 76,316     $ 27,095
Amortization of core deposit intangible assets 26 $ 158 85 $ 483    
Aggregate mortgage servicing rights amortization $ 22 $ 23 65 $ 51    
UCB [Member]            
Acquired Finite-Lived Intangible Assets [Line Items]            
Goodwill         $ 49,221  
Increased in carrying amount of goodwill     $ 49,221