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Allowance for Loan losses - Impaired Loans and Related Allowance (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | $ 4,575 | $ 5,545
Recorded Investment with No Allowance | 3,811 | 4,707
Recorded Investment with Allowance | 0 | 153
Total Recorded Investment | 3,811 | 4,860
Related Allowance | 0 | 37
Average Recorded Investment | 4,656 | 7,034
Interest Income Recognized | 232 | 176
Real estate loans | Construction and land development |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 1,627 | 2,388
Recorded Investment with No Allowance | 1,150 | 1,973
Recorded Investment with Allowance | 0 | 0
Total Recorded Investment | 1,150 | 1,973
Related Allowance | 0 | 0
Average Recorded Investment | 1,814 | 2,407
Interest Income Recognized | 63 | 66
Real estate loans | Secured by 1-4 family residential |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 1,387 | 1,851
Recorded Investment with No Allowance | 1,307 | 1,675
Recorded Investment with Allowance | 0 | 153
Total Recorded Investment | 1,307 | 1,828
Related Allowance | 0 | 37
Average Recorded Investment | 1,637 | 2,013
Interest Income Recognized | 64 | 87
Real estate loans | Other real estate |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 1,483 | 1,213
Recorded Investment with No Allowance | 1,289 | 984
Recorded Investment with Allowance | 0 | 0
Total Recorded Investment | 1,289 | 984
Related Allowance | 0 | 0
Average Recorded Investment | 1,137 | 2,529
Interest Income Recognized | 95 | 22
Commercial and industrial loans |  |
Financing Receivable, Impaired [Line Items] |  |
Unpaid Principal Balance | 78 | 93
Recorded Investment with No Allowance | 65 | 75
Recorded Investment with Allowance | 0 | 0
Total Recorded Investment | 65 | 75
Related Allowance | 0 | 0
Average Recorded Investment | 68 | 85
Interest Income Recognized | $ 10 | $ 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=84173941&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=84173941&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=84173941&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=84173941&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=84173941&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=84173941&loc=d3e5212-111524 + Details
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