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Goodwill and Other Intangible Assets - Summary of Amortizable Intangible Assets (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]    
Balance of intangible assets, Gross CDI Carrying Amount, Beginning of period $ 37,940 $ 34,089
Additions due to acquisitions 3,157 3,851
Balance of intangible assets, Gross CDI Carrying Amount, End of period 41,097 37,940
Aggregate amortization expense, Gross CDI Carrying Amount 1,329 1,106
Estimated Amortization Expense:    
Estimated Amortization Expense, 2018 1,304  
Estimated Amortization Expense, 2019 1,067  
Estimated Amortization Expense, 2020 718  
Estimated Amortization Expense, 2021 703  
Estimated Amortization Expense, Thereafter 3,046  
Balance of intangible assets, Accumulated Amortization, Beginning of period (32,930) (31,824)
Balance of intangible assets, Accumulated Amortization, End of period (34,259) (32,930)
Balance of intangible assets, Net CDI Amount, Beginning of period 5,010 2,265
Balance of intangible assets, Net CDI Amount, End of period $ 6,838 $ 5,010