Allowance for Loan Losses - Ending Allocation of Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 246		$ 188
Collectively evaluated for impairment	12,567		12,942
Total	12,946	$ 13,047	13,305	$ 13,451	$ 14,547	$ 14,361
Individually evaluated for impairment	4,258		6,539
Collectively evaluated for impairment	1,137,506		1,048,711
Total	1,141,992		1,055,506
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	133		175
Loan balance of loans acquired with credit deterioration	228		256
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	122		82
Collectively evaluated for impairment	1,408		1,850
Total	1,611	1,608	2,018	1,710	1,557	1,478
Individually evaluated for impairment	1,170		1,983
Collectively evaluated for impairment	146,278		133,391
Total	147,537		135,462
Commercial and Agriculture [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	81		86
Loan balance of loans acquired with credit deterioration	89		88
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	4		4
Collectively evaluated for impairment	2,069		2,167
Total	2,073	2,010	2,171	2,310	2,393	2,467
Individually evaluated for impairment	1,039		1,896
Collectively evaluated for impairment	166,639		159,468
Total	167,678		161,364
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	5,273		4,606
Total	5,273	4,739	4,606	4,543	4,969	4,657
Individually evaluated for impairment	50		359
Collectively evaluated for impairment	424,380		395,572
Total	424,430		395,931
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	114		102
Collectively evaluated for impairment	2,313		2,898
Total	2,479	2,676	3,089	3,455	3,899	4,086
Individually evaluated for impairment	1,386		1,686
Collectively evaluated for impairment	266,314		245,454
Total	267,839		247,308
Residential Real Estate [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	52		89
Loan balance of loans acquired with credit deterioration	139		168
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	667		420
Total	667	482	420	424	366	371
Collectively evaluated for impairment	77,978		56,293
Total	77,978		56,293
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	6
Collectively evaluated for impairment	418		442
Total	424	425	442	430	476	538
Individually evaluated for impairment	613		614
Collectively evaluated for impairment	38,353		40,556
Total	38,966		41,170
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	325		314
Total	325	324	314	337	358	382
Individually evaluated for impairment			1
Collectively evaluated for impairment	17,564		17,977
Total	17,564		17,978
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	94		245
Total	$ 94	$ 783	$ 245	$ 242	$ 529	$ 382