Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 14,551	$ 25,777	$ 15,376
Recorded Investment with No Allowance	8,827	16,102	9,039
Recorded Investment with Allowance	4,207	5,136	4,883
Total Recorded Investment	13,034	21,238	13,922
Related Allowance	1,901	1,538	1,907
Average Recorded Investment	13,587	21,279	19,239
Interest Income Recognized	79	121	385
Construction and Land Development [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,198	8,864	3,299
Recorded Investment with No Allowance	2,196	4,018	2,800
Recorded Investment with Allowance	875	2,550	405
Total Recorded Investment	3,071	6,568	3,205
Related Allowance	341	862	245
Average Recorded Investment	3,171	6,684	5,532
Interest Income Recognized	16	11	40
Secured by 1-4 Family Residential [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	3,672	4,391	4,327
Recorded Investment with No Allowance	2,509	2,932	2,526
Recorded Investment with Allowance	238	536	888
Total Recorded Investment	2,747	3,468	3,414
Related Allowance	85	126	173
Average Recorded Investment	3,149	3,313	3,433
Interest Income Recognized	29	31	138
Other Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	7,559	12,346	7,623
Recorded Investment with No Allowance	4,119	9,139	3,708
Recorded Investment with Allowance	2,984	1,891	3,475
Total Recorded Investment	7,103	11,030	7,183
Related Allowance	1,447	505	1,456
Average Recorded Investment	7,149	11,082	10,115
Interest Income Recognized	34	79	206
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	122	176	127
Recorded Investment with No Allowance	3	13	5
Recorded Investment with Allowance	110	159	115
Total Recorded Investment	113	172	120
Related Allowance	28	45	33
Average Recorded Investment	118	200	159
Interest Income Recognized			$ 1

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.

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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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