Loans and Allowance For Loan Losses - Categories of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Notes And Loans Receivable [Line Items]
Total loans	$ 2,575,408	$ 2,117,270
Allowance for loan losses	(11,454)	(8,498)	$ (6,432)	$ (5,506)
Net loans	2,563,954	2,108,772
Commercial Real Estate [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	1,251,992	987,661
Allowance for loan losses	(4,662)	(2,740)	(2,420)	(2,051)
Commercial and Industrial [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	582,527	521,510
Allowance for loan losses	(2,707)	(2,136)	(1,881)	(1,366)
Residential Real Estate [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	444,540	376,705
Allowance for loan losses	(2,320)	(2,262)	(1,765)	(1,824)
Agricultural Real Estate [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	139,332	86,486
Allowance for loan losses	(391)	(319)	(35)	(29)
Consumer [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	62,894	49,361
Allowance for loan losses	(1,070)	(768)	(266)	(187)
Agricultural [Member]
Accounts Notes And Loans Receivable [Line Items]
Total loans	94,123	95,547
Allowance for loan losses	$ (304)	$ (273)	$ (65)	$ (49)

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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