Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 21,880	$ 22,675
Recorded Investment with No Allowance	12,838	7,198
Recorded Investment with Allowance	4,752	9,782
Total Recorded Investment	17,590	16,980
Related Allowance	1,081	1,838
Average Recorded Investment	14,754	20,990
Interest Income Recognized	528	548
Construction and Land Development [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	9,441	2,947
Recorded Investment with No Allowance	5,406	622
Recorded Investment with Allowance	1,811	1,894
Total Recorded Investment	7,217	2,516
Related Allowance	743	567
Average Recorded Investment	4,469	5,691
Interest Income Recognized	217	99
Secured by 1-4 Family Residential [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	4,339	4,706
Recorded Investment with No Allowance	2,533	1,690
Recorded Investment with Allowance	906	2,086
Total Recorded Investment	3,439	3,776
Related Allowance	81	306
Average Recorded Investment	2,673	4,821
Interest Income Recognized	105	163
Other Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	7,855	14,861
Recorded Investment with No Allowance	4,883	4,886
Recorded Investment with Allowance	1,807	5,642
Total Recorded Investment	6,690	10,528
Related Allowance	201	930
Average Recorded Investment	6,817	10,148
Interest Income Recognized	197	276
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	245	161
Recorded Investment with No Allowance	16
Recorded Investment with Allowance	228	160
Total Recorded Investment	244	160
Related Allowance	56	35
Average Recorded Investment	795	330
Interest Income Recognized	9	10
Consumer and Other Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance
Recorded Investment with No Allowance
Recorded Investment with Allowance
Total Recorded Investment
Related Allowance
Average Recorded Investment
Interest Income Recognized

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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=28368275&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=28368275&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

+ Details

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- Definition

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 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953550-111524

+ Details

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- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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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

+ Details

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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

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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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