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Allowance for Loan Losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 4,698	$ 5,512	$ 4,575
Recorded Investment with No Allowance	4,481	4,383	3,811
Recorded Investment with Allowance	0	424	0
Total Recorded Investment	4,481	4,807	3,811
Related Allowance	0	57	0
Average Recorded Investment	3,568	4,651	4,656
Interest Income Recognized	87	108	232
Real estate loans | Construction and Land Development
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	984	2,066	1,627
Recorded Investment with No Allowance	865	1,613	1,150
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	865	1,613	1,150
Related Allowance	0	0	0
Average Recorded Investment	1,063	1,955	1,814
Interest Income Recognized	27	26	63
Real estate loans | Secured by 1-4 Family Residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,415	1,820	1,387
Recorded Investment with No Allowance	1,330	1,348	1,307
Recorded Investment with Allowance	0	424	0
Total Recorded Investment	1,330	1,772	1,307
Related Allowance	0	57	0
Average Recorded Investment	1,300	1,667	1,637
Interest Income Recognized	28	31	64
Real estate loans | Other real estate loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,029	1,540	1,483
Recorded Investment with No Allowance	2,029	1,355	1,289
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	2,029	1,355	1,289
Related Allowance	0	0	0
Average Recorded Investment	1,140	958	1,137
Interest Income Recognized	30	49	95
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	270	86	78
Recorded Investment with No Allowance	257	67	65
Recorded Investment with Allowance	0	0	0
Total Recorded Investment	257	67	65
Related Allowance	0	0	0
Average Recorded Investment	65	71	68
Interest Income Recognized	$ 2	$ 2	$ 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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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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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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-Topic 310
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-Section 50
-Paragraph 14A
-Subparagraph (b)
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