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Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	41	65	61
Extend Maturity	$ 7,081	$ 1,521	$ 1,754
Modify Rate	2,095	6,614	3,658
Modify Payments	1,878	6,523	6,440
Total Pre-Modification Outstanding Recorded Investment	11,054	14,658	11,852
Post-Modification Outstanding Recorded Investment	8,274	10,275	10,956
Specific Reserve	$ 596	$ 500	$ 1,332
Commercial Financial Agricultural And Other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	6	5	12
Extend Maturity	$ 6,768	$ 23	$ 1,751
Modify Rate	1,806	6,318	3,195
Modify Payments	987	3,854	4,527
Total Pre-Modification Outstanding Recorded Investment	9,561	10,195	9,473
Post-Modification Outstanding Recorded Investment	6,946	6,210	8,823
Specific Reserve	$ 566	$ 317	$ 1,330
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	20	39	32
Extend Maturity	$ 134	$ 107	$ 0
Modify Rate	261	214	296
Modify Payments	573	2,619	1,414
Total Pre-Modification Outstanding Recorded Investment	968	2,940	1,710
Post-Modification Outstanding Recorded Investment	851	2,698	1,575
Specific Reserve	$ 1	$ 124	$ 2
Commercial real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	5	8	1
Extend Maturity	$ 179	$ 1,368	$ 0
Modify Rate	0	0	0
Modify Payments	269	25	464
Total Pre-Modification Outstanding Recorded Investment	448	1,393	464
Post-Modification Outstanding Recorded Investment	412	1,271	389
Specific Reserve	$ 29	$ 59	$ 0
Loans To Individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	10	13	16
Extend Maturity	$ 0	$ 23	$ 3
Modify Rate	28	82	167
Modify Payments	49	25	35
Total Pre-Modification Outstanding Recorded Investment	77	130	205
Post-Modification Outstanding Recorded Investment	65	96	169
Specific Reserve	$ 0	$ 0	$ 0

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Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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