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Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 8,456	$ 15,376
Recorded Investment with No Allowance	6,784	9,039
Recorded Investment with Allowance	921	4,883
Total Recorded Investment	7,705	13,922
Related Allowance	544	1,907
Average Recorded Investment	10,544	19,239
Interest Income Recognized	225	385
Construction and Land Development [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,741	3,299
Recorded Investment with No Allowance	2,206	2,800
Recorded Investment with Allowance	338	405
Total Recorded Investment	2,544	3,205
Related Allowance	326	245
Average Recorded Investment	2,967	5,532
Interest Income Recognized	60	40
Secured by 1-4 Family Residential [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,116	4,327
Recorded Investment with No Allowance	2,021	2,526
Recorded Investment with Allowance	23	888
Total Recorded Investment	2,044	3,414
Related Allowance	23	173
Average Recorded Investment	2,526	3,433
Interest Income Recognized	107	138
Other Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,492	7,623
Recorded Investment with No Allowance	2,463	3,708
Recorded Investment with Allowance	560	3,475
Total Recorded Investment	3,023	7,183
Related Allowance	195	1,456
Average Recorded Investment	4,933	10,115
Interest Income Recognized	58	206
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	107	127
Recorded Investment with No Allowance	94	5
Recorded Investment with Allowance		115
Total Recorded Investment	94	120
Related Allowance		33
Average Recorded Investment	$ 118	159
Interest Income Recognized		$ 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=49124121&loc=SL6953423-111524

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ 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=49124121&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=49124121&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=49124121&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=49124121&loc=d3e5212-111524

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- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without 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 (a)(4)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

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- Definition

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=49124121&loc=SL6953550-111524

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- Definition

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=49124121&loc=SL6953550-111524

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