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Loans and Allowance for Loan Losses - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Loan
Dec. 31, 2017
USD ($)
Pool
Loan
Dec. 31, 2016
USD ($)
Accounts Notes And Loans Receivable [Line Items]      
Loans $ 2,563,954,000 $ 2,108,772,000  
Loans purchased at discount 827,676,000 796,064,000 $ 388,251,000
Discount associated with loans purchased 11,372,000 7,231,000 3,596,000
Provision for loan losses $ 3,961,000 $ 2,953,000 2,119,000
Troubled debt restructurings | Loan 0 0  
Consumer Loans [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Over-draft deposit accounts $ 1,279,000 $ 741,000  
Purchased Credit Impaired Loans [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Accretable yield 3,785,000 1,980,000 1,063,000
Interest income recognized 1,096,000 1,785,000 1,237,000
Provision for loan losses 714,000 194,000 $ 0
Residential Real Estate Loan Pools [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Loans $ 64,558,000 85,868,000  
Payments to acquire loans   $ 14,767,000  
Purchased pools of residential real estate | Pool   1