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Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) Contract	Dec. 31, 2016 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	58	41	65
Extend Maturity	$ 316	$ 7,081	$ 1,521
Modify Rate	330	2,095	6,614
Modify Payments	10,635	1,878	6,523
Total Pre-Modification Outstanding Recorded Investment	11,281	11,054	14,658
Post-Modification Outstanding Recorded Investment	8,829	8,274	10,275
Specific Reserve	$ 0	$ 596	$ 500
Commercial Financial Agricultural And Other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	3	6	5
Extend Maturity	$ 74	$ 6,768	$ 23
Modify Rate	0	1,806	6,318
Modify Payments	8,250	987	3,854
Total Pre-Modification Outstanding Recorded Investment	8,324	9,561	10,195
Post-Modification Outstanding Recorded Investment	6,104	6,946	6,210
Specific Reserve	$ 0	$ 566	$ 317
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	37	20	39
Extend Maturity	$ 242	$ 134	$ 107
Modify Rate	241	261	214
Modify Payments	1,316	573	2,619
Total Pre-Modification Outstanding Recorded Investment	1,799	968	2,940
Post-Modification Outstanding Recorded Investment	1,638	851	2,698
Specific Reserve	$ 0	$ 1	$ 124
Commercial real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	3	5	8
Extend Maturity	$ 0	$ 179	$ 1,368
Modify Rate	0	0	0
Modify Payments	1,016	269	25
Total Pre-Modification Outstanding Recorded Investment	1,016	448	1,393
Post-Modification Outstanding Recorded Investment	975	412	1,271
Specific Reserve	$ 0	$ 29	$ 59
Loans To Individuals [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts | Contract	15	10	13
Extend Maturity	$ 0	$ 0	$ 23
Modify Rate	89	28	82
Modify Payments	53	49	25
Total Pre-Modification Outstanding Recorded Investment	142	77	130
Post-Modification Outstanding Recorded Investment	112	65	96
Specific Reserve	$ 0	$ 0	$ 0

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
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