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Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 3,546	$ 4,575
Recorded Investment with No Allowance	2,802	3,811
Recorded Investment with Allowance	634	0
Total Recorded Investment	3,436	3,811
Related Allowance	243	0
Average Recorded Investment	3,455	4,656
Interest Income Recognized	85	232
Real estate loans | Construction and land development
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	336	1,627
Recorded Investment with No Allowance	0	1,150
Recorded Investment with Allowance	327	0
Total Recorded Investment	327	1,150
Related Allowance	71	0
Average Recorded Investment	758	1,814
Interest Income Recognized	12	63
Real estate loans | Secured by 1-4 family residential
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	720	1,387
Recorded Investment with No Allowance	356	1,307
Recorded Investment with Allowance	307	0
Total Recorded Investment	663	1,307
Related Allowance	172	0
Average Recorded Investment	966	1,637
Interest Income Recognized	22	64
Real estate loans | Other real estate
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,290	1,483
Recorded Investment with No Allowance	2,249	1,289
Recorded Investment with Allowance	0	0
Total Recorded Investment	2,249	1,289
Related Allowance	0	0
Average Recorded Investment	1,585	1,137
Interest Income Recognized	51	95
Commercial and industrial loans
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	200	78
Recorded Investment with No Allowance	197	65
Recorded Investment with Allowance	0	0
Total Recorded Investment	197	65
Related Allowance	0	0
Average Recorded Investment	146	68
Interest Income Recognized	$ 0	$ 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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- 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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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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