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Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | $ 5,545 | $ 8,456
Recorded Investment with No Allowance | 4,707 | 6,784
Recorded Investment with Allowance | 153 | 921
Total Recorded Investment | 4,860 | 7,705
Related Allowance | 37 | 544
Average Recorded Investment | 7,034 | 10,544
Interest Income Recognized | 176 | 225
Construction and Land Development [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 2,388 | 2,741
Recorded Investment with No Allowance | 1,973 | 2,206
Recorded Investment with Allowance |  | 338
Total Recorded Investment | 1,973 | 2,544
Related Allowance |  | 326
Average Recorded Investment | 2,407 | 2,967
Interest Income Recognized | 66 | 60
Secured by 1-4 Family Residential [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 1,851 | 2,116
Recorded Investment with No Allowance | 1,675 | 2,021
Recorded Investment with Allowance | 153 | 23
Total Recorded Investment | 1,828 | 2,044
Related Allowance | 37 | 23
Average Recorded Investment | 2,013 | 2,526
Interest Income Recognized | 87 | 107
Other Real Estate Loans [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 1,213 | 3,492
Recorded Investment with No Allowance | 984 | 2,463
Recorded Investment with Allowance |  | 560
Total Recorded Investment | 984 | 3,023
Related Allowance |  | 195
Average Recorded Investment | 2,529 | 4,933
Interest Income Recognized | 22 | 58
Commercial and Industrial [Member] |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 93 | 107
Recorded Investment with No Allowance | 75 | 94
Total Recorded Investment | 75 | 94
Average Recorded Investment | 85 | $ 118
Interest Income Recognized | $ 1 |

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- 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=66008989&loc=SL6953423-111524 + Details
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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. + Details
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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=66008989&loc=SL6953423-111524 + Details
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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=66008989&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=66008989&loc=d3e5258-111524 + Details
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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=66008989&loc=d3e5212-111524 + 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 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=66008989&loc=d3e5212-111524 + Details
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