Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 25,777	$ 22,292	$ 26,072
Recorded Investment with No Allowance	16,102	7,657	16,343
Recorded Investment with Allowance	5,136	11,563	5,351
Total Recorded Investment	21,238	19,220	21,694
Related Allowance	1,538	1,924	1,379
Average Recorded Investment	21,279	17,696	16,009
Interest Income Recognized	121	136	805
Construction and Land Development [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	8,864	6,441	9,086
Recorded Investment with No Allowance	4,018	4,988	4,259
Recorded Investment with Allowance	2,550	1,110	2,603
Total Recorded Investment	6,568	6,098	6,862
Related Allowance	862	604	882
Average Recorded Investment	6,684	4,244	5,397
Interest Income Recognized	11	16	204
Secured by 1-4 Family Residential [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	4,391	3,898	4,341
Recorded Investment with No Allowance	2,932	1,024	2,515
Recorded Investment with Allowance	536	1,935	916
Total Recorded Investment	3,468	2,959	3,431
Related Allowance	126	156	190
Average Recorded Investment	3,313	4,116	2,864
Interest Income Recognized	31	23	146
Other Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	12,346	9,443	12,385
Recorded Investment with No Allowance	9,139	1,645	9,455
Recorded Investment with Allowance	1,891	6,009	1,688
Total Recorded Investment	11,030	7,654	11,143
Related Allowance	505	903	263
Average Recorded Investment	11,082	9,151	7,079
Interest Income Recognized	79	70	441
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	176	2,510	260
Recorded Investment with No Allowance	13		114
Recorded Investment with Allowance	159	2,509	144
Total Recorded Investment	172	2,509	258
Related Allowance	45	261	44
Average Recorded Investment	200	185	669
Interest Income Recognized		27	14
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

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant