Loans - Analysis of Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	$ 7,095	$ 7,001	$ 7,001
Charge Offs	(145)	(44)	(288)
Recoveries	37	37	150
Provision (Credit)	40	73	222
Other Non-interest expense related to unfunded	38	2
Ending Balance	7,065	7,069	7,095
Ending balance: individually evaluated for impairment	104	117
Ending balance: collectively evaluated for impairment	6,961	6,952
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	834,737	771,206	823,024
Ending balance: individually evaluated for impairment	1,606	1,793
Ending balance: collectively evaluated for impairment	833,131	769,413
Agricultural Real Estate [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	244	241	241
Provision (Credit)	19	3
Ending Balance	263	244	244
Ending balance: collectively evaluated for impairment	263	244
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	67,569	62,803	64,044
Ending balance: individually evaluated for impairment		101
Ending balance: collectively evaluated for impairment	67,569	62,702
Agricultural [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	667	616	616
Recoveries	3	1
Provision (Credit)	36	17
Ending Balance	706	634	667
Ending balance: collectively evaluated for impairment	706	634
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	99,954	87,078	95,238
Ending balance: collectively evaluated for impairment	99,954	87,078
Commercial And Industrial and Industrial Development Bonds [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	1,546	1,318	1,318
Recoveries	2	3
Provision (Credit)	(105)	(22)
Ending Balance	1,443	1,299	1,546
Ending balance: individually evaluated for impairment	42	33
Ending balance: collectively evaluated for impairment	1,401	1,266
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	129,865	121,183	132,787
Ending balance: individually evaluated for impairment	419	115
Ending balance: collectively evaluated for impairment	129,446	121,068
Unfunded Loan Commitment & Letters of Credit [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	227	217	217
Other Non-interest expense related to unfunded	38	2
Ending Balance	265	219	227
Ending balance: collectively evaluated for impairment	265	219
Ending balance: loans acquired with deteriorated credit quality	0	0
Receivables Acquired with Deteriorated Credit Quality [Member]
FINANCING RECEIVABLES:
Ending balance: loans acquired with deteriorated credit quality	121	198
Consumer Real Estate [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	343	316	316
Charge Offs	(34)
Recoveries		10
Provision (Credit)	(55)	(49)
Ending Balance	254	277	343
Ending balance: individually evaluated for impairment	20	23
Ending balance: collectively evaluated for impairment	234	254
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	84,052	84,081	83,200
Ending balance: individually evaluated for impairment	569	1,081
Ending balance: collectively evaluated for impairment	83,483	83,000
Consumer Real Estate [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
FINANCING RECEIVABLES:
Ending balance: loans acquired with deteriorated credit quality	121	198
Commercial Real Estate [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	3,149	3,250	3,250
Charge Offs	(14)
Recoveries	2	2
Provision (Credit)	537	(244)
Ending Balance	3,674	3,008	3,149
Ending balance: individually evaluated for impairment	42	61
Ending balance: collectively evaluated for impairment	3,632	2,947
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	414,666	382,183	409,953
Ending balance: individually evaluated for impairment	618	496
Ending balance: collectively evaluated for impairment	414,048	381,687
Consumer [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	441	394	394
Charge Offs	(97)	(44)
Recoveries	30	21
Provision (Credit)	57	26
Ending Balance	431	397	441
Ending balance: collectively evaluated for impairment	431	397
Ending balance: loans acquired with deteriorated credit quality	0	0
FINANCING RECEIVABLES:
Ending balance	38,631	33,878	37,802
Ending balance: collectively evaluated for impairment	38,631	33,878
Unallocated [Member]
ALLOWANCE FOR CREDIT LOSSES:
Beginning balance	478	649	649
Provision (Credit)	(449)	342
Ending Balance	29	991	$ 478
Ending balance: collectively evaluated for impairment	29	991
Ending balance: loans acquired with deteriorated credit quality	$ 0	$ 0

Allowance For Credit Losses [Abstract]

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Financing receivable allowance for credit losses acquired with deteriorated credit quality ending period.

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Financing Receivables [Abstract]

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Other non interest Income expense (recovery) related to unfunded.

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount before allowance and after 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.

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Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

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