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Allowance for Loan Losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 3,123	$ 5,831	$ 4,575
Recorded Investment with No Allowance	2,513	4,995	3,811
Recorded Investment with Allowance	494	0	0
Total Recorded Investment	3,007	4,995	3,811
Related Allowance	107	0	0
Average Recorded Investment	3,634	4,755	4,656
Interest Income Recognized	54	159	232
Real estate loans | Construction and Land Development
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	343	2,429	1,627
Recorded Investment with No Allowance	0	1,959	1,150
Recorded Investment with Allowance	343	0	0
Total Recorded Investment	343	1,959	1,150
Related Allowance	87	0	0
Average Recorded Investment	900	1,904	1,814
Interest Income Recognized	12	40	63
Real estate loans | Secured by 1-4 Family Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	511	1,808	1,387
Recorded Investment with No Allowance	287	1,654	1,307
Recorded Investment with Allowance	151	0	0
Total Recorded Investment	438	1,654	1,307
Related Allowance	20	0	0
Average Recorded Investment	1,172	1,697	1,637
Interest Income Recognized	5	47	64
Real estate loans | Other real estate loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,069	1,512	1,483
Recorded Investment with No Allowance	2,029	1,319	1,289
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	2,029	1,319	1,289
Related Allowance	0	0	0
Average Recorded Investment	1,433	1,084	1,137
Interest Income Recognized	37	68	95
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	200	82	78
Recorded Investment with No Allowance	197	63	65
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	197	63	65
Related Allowance	0	0	0
Average Recorded Investment	129	70	68
Interest Income Recognized	$ 0	$ 4	$ 10

X

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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+ References

No definition available.

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The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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+ References

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-Name Accounting Standards Codification
-Topic 310
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-Section 50
-Paragraph 14A
-Subparagraph (b)
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