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Loans - Contractual Aging of Recorded Investment in Past Due Loans by Portfolio Classification of Loans (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | $ 1,114 | $ 1,744 |
Current | 821,910 | 756,350 |
Loans after net deferred loan fees and costs | 823,024 | 758,094 |
Recorded Investment > 90 Days and Accruing | 0 | 0 | $ 0
Agricultural Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 101 | 144 |
Current | 63,943 | 62,192 |
Loans after net deferred loan fees and costs | 64,044 | 62,336 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
Agricultural [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due |  | 101 |
Current | 95,238 | 84,591 |
Loans after net deferred loan fees and costs | 95,238 | 84,692 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
Commercial And Industrial and Industrial Development Bonds [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 42 |  |
Current | 132,745 | 115,093 |
Loans after net deferred loan fees and costs | 132,787 | 115,093 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
Consumer Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 890 | 1,404 |
Current | 82,310 | 84,469 |
Loans after net deferred loan fees and costs | 83,200 | 85,873 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
Commercial Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 38 | 60 |
Current | 409,915 | 376,827 |
Loans after net deferred loan fees and costs | 409,953 | 376,887 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
Consumer [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 43 | 35 |
Current | 37,759 | 33,178 |
Loans after net deferred loan fees and costs | 37,802 | 33,213 |
Recorded Investment > 90 Days and Accruing | 0 | 0 |
30 to 59 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 599 | 1,084 |
30 to 59 Days Past Due [Member] | Agricultural Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due |  | 12 |
30 to 59 Days Past Due [Member] | Agricultural [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due |  | 101 |
30 to 59 Days Past Due [Member] | Consumer Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 565 | 882 |
30 to 59 Days Past Due [Member] | Commercial Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due |  | 60 |
30 to 59 Days Past Due [Member] | Consumer [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 34 | 29 |
60 to 89 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 256 | 21 |
60 to 89 Days Past Due [Member] | Commercial And Industrial and Industrial Development Bonds [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 42 |  |
60 to 89 Days Past Due [Member] | Consumer Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 212 | 15 |
60 to 89 Days Past Due [Member] | Consumer [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 2 | 6 |
Greater Than 90 Days [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 259 | 639 |
Greater Than 90 Days [Member] | Agricultural Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 101 | 132 |
Greater Than 90 Days [Member] | Consumer Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 113 | $ 507 |
Greater Than 90 Days [Member] | Commercial Real Estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | 38 |  |
Greater Than 90 Days [Member] | Consumer [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total Past Due | $ 7 |  |

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