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Goodwill and Intangible Assets - Amortizable Intangible Assets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]    
Amortizable intangible assets Core deposit intangible, Gross Carrying Amount $ 12,400  
Core Deposits [Member]    
Finite-Lived Intangible Assets [Line Items]    
Amortizable intangible assets Core deposit intangible, Gross Carrying Amount 20,711 $ 16,151
Amortizable intangible assets Core deposit intangible, Accumulated Amortization $ (8,309) $ (6,785)