Allowance for Loan Losses - Ending Allocation of Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 117			$ 263
Collectively evaluated for impairment	13,237			13,975
Total	13,451	$ 14,547	$ 14,361	14,361	$ 14,760	$ 14,707	$ 14,268
Individually evaluated for impairment	7,214			7,354
Collectively evaluated for impairment	1,039,487			993,910
Total	1,046,967			1,001,527
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	97			123
Loan balance of loans acquired with credit deterioration	266			263
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment				23
Collectively evaluated for impairment	1,710			1,455
Total	1,710	1,557	1,478	1,478	1,378	1,395	1,819
Individually evaluated for impairment	2,386			873
Collectively evaluated for impairment	127,225			123,397
Total	129,700			124,402
Commercial and Agriculture [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan balance of loans acquired with credit deterioration	89			132
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	4,543			4,657
Total	4,543	4,969	4,657	4,657	4,939	3,153	4,334
Individually evaluated for impairment	369			1,742
Collectively evaluated for impairment	385,494			346,697
Total	385,863			348,439
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	4			103
Collectively evaluated for impairment	2,306			2,364
Total	2,310	2,393	2,467	2,467	2,682	5,073	2,221
Individually evaluated for impairment	1,939			2,141
Collectively evaluated for impairment	165,625			165,756
Total	167,564			167,897
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	113			137
Collectively evaluated for impairment	3,245			3,826
Total	3,455	3,899	4,086	4,086	4,222	3,470	3,747
Individually evaluated for impairment	1,854			1,642
Collectively evaluated for impairment	245,143			234,565
Total	247,174			236,338
Residential Real Estate [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	97			123
Loan balance of loans acquired with credit deterioration	177			131
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	424			371
Total	424	366	371	371	428	434	428
Collectively evaluated for impairment	56,919			58,898
Total	56,919			58,898
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	430			538
Total	430	476	538	538	658	584	822
Individually evaluated for impairment	665			953
Collectively evaluated for impairment	41,197			46,040
Total	41,862			46,993
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	337			382
Total	337	358	382	382	374	190	200
Individually evaluated for impairment	1			3
Collectively evaluated for impairment	17,884			18,557
Total	17,885			18,560
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	242			382
Total	$ 242	$ 529	$ 382	$ 382	$ 79	$ 408	$ 697