Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 15,376	$ 26,072	$ 22,675
Recorded Investment with No Allowance	9,039	16,343	7,198
Recorded Investment with Allowance	4,883	5,351	9,782
Total Recorded Investment	13,922	21,694	16,980
Related Allowance	1,907	1,379	1,838
Average Recorded Investment	19,239	16,009	20,990
Interest Income Recognized	385	805	548
Construction and Land Development [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,299	9,086	2,947
Recorded Investment with No Allowance	2,800	4,259	622
Recorded Investment with Allowance	405	2,603	1,894
Total Recorded Investment	3,205	6,862	2,516
Related Allowance	245	882	567
Average Recorded Investment	5,532	5,397	5,691
Interest Income Recognized	40	204	99
Secured by 1-4 Family Residential [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	4,327	4,341	4,706
Recorded Investment with No Allowance	2,526	2,515	1,690
Recorded Investment with Allowance	888	916	2,086
Total Recorded Investment	3,414	3,431	3,776
Related Allowance	173	190	306
Average Recorded Investment	3,433	2,864	4,821
Interest Income Recognized	138	146	163
Other Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	7,623	12,385	14,861
Recorded Investment with No Allowance	3,708	9,455	4,886
Recorded Investment with Allowance	3,475	1,688	5,642
Total Recorded Investment	7,183	11,143	10,528
Related Allowance	1,456	263	930
Average Recorded Investment	10,115	7,079	10,148
Interest Income Recognized	206	441	276
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	127	260	161
Recorded Investment with No Allowance	5	114
Recorded Investment with Allowance	115	144	160
Total Recorded Investment	120	258	160
Related Allowance	33	44	35
Average Recorded Investment	159	669	330
Interest Income Recognized	$ 1	$ 14	$ 10

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

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=35738203&loc=SL6953423-111524

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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=35738203&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=35738203&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=35738203&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

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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

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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=35738203&loc=SL6953550-111524

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Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

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