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Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 4,575	$ 5,545
Recorded Investment with No Allowance	3,811	4,707
Recorded Investment with Allowance	0	153
Total Recorded Investment	3,811	4,860
Related Allowance	0	37
Average Recorded Investment	4,656	7,034
Interest Income Recognized	232	176
Real estate loans | Construction and land development
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,627	2,388
Recorded Investment with No Allowance	1,150	1,973
Recorded Investment with Allowance	0	0
Total Recorded Investment	1,150	1,973
Related Allowance	0	0
Average Recorded Investment	1,814	2,407
Interest Income Recognized	63	66
Real estate loans | Secured by 1-4 family residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,387	1,851
Recorded Investment with No Allowance	1,307	1,675
Recorded Investment with Allowance	0	153
Total Recorded Investment	1,307	1,828
Related Allowance	0	37
Average Recorded Investment	1,637	2,013
Interest Income Recognized	64	87
Real estate loans | Other real estate
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	1,483	1,213
Recorded Investment with No Allowance	1,289	984
Recorded Investment with Allowance	0	0
Total Recorded Investment	1,289	984
Related Allowance	0	0
Average Recorded Investment	1,137	2,529
Interest Income Recognized	95	22
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	78	93
Recorded Investment with No Allowance	65	75
Recorded Investment with Allowance	0	0
Total Recorded Investment	65	75
Related Allowance	0	0
Average Recorded Investment	68	85
Interest Income Recognized	$ 10	$ 1

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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- Definition

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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Amount of investment in impaired financing receivables for which there is no 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 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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Amount of investment in impaired financing receivables for which there is 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 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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