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Loans - Loans Individually Evaluated for Impairment by Portfolio Class of Loans (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Recorded Investment | $ 1,131 | $ 1,141 |
With a specific allowance recorded, Recorded Investment | 614 | 711 |
Related Allowance | 106 | 135 |
Recorded Investment | 1,745 | 1,852 |
Related Allowance | 106 | 135 |
Average Recorded Investment | 1,885 | 1,802 | $ 2,509
Interest Income Recognized | 57 | 64 | 96
Interest Income Recognized Cash Basis | 23 | 27 | $ 60
Agricultural Real Estate [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Recorded Investment | 101 | 132 |
With no related allowance recorded, Unpaid Principal Balance | 101 | 132 |
With no related allowance recorded, Average Recorded Investment | 123 | 147 |
With a specific allowance recorded, Average Recorded Investment |  | 9 |
Recorded Investment | 101 | 132 |
Unpaid Principal Balance | 101 | 132 |
Average Recorded Investment | 123 | 156 |
Agricultural [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Average Recorded Investment | 27 |  |
Average Recorded Investment | 27 |  |
Consumer Real Estate [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Recorded Investment | 495 | 1,009 |
With no related allowance recorded, Unpaid Principal Balance | 495 | 1,009 |
With no related allowance recorded, Average Recorded Investment | 911 | 101 |
With no related allowance recorded, Interest Income Recognized | 30 | 16 |
With no related allowance recorded, Interest Income Recognized Cash Basis | 21 | 4 |
With a specific allowance recorded, Recorded Investment | 80 | 94 |
With a specific allowance recorded, Unpaid Principal Balance | 80 | 94 |
Related Allowance | 21 | 34 |
With a specific allowance recorded, Average Recorded Investment | 88 | 344 |
Recorded Investment | 575 | 1,103 |
Unpaid Principal Balance | 575 | 1,103 |
Related Allowance | 21 | 34 |
Average Recorded Investment | 999 | 445 |
Interest Income Recognized | 30 | 16 |
Interest Income Recognized Cash Basis | 21 | 4 |
Commercial Real Estate [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Recorded Investment | 202 |  |
With no related allowance recorded, Unpaid Principal Balance | 202 |  |
With no related allowance recorded, Average Recorded Investment | 51 | 427 |
With no related allowance recorded, Interest Income Recognized | 1 | 24 |
With no related allowance recorded, Interest Income Recognized Cash Basis |  | 23 |
With a specific allowance recorded, Recorded Investment | 423 | 501 |
With a specific allowance recorded, Unpaid Principal Balance | 423 | 501 |
Related Allowance | 46 | 66 |
With a specific allowance recorded, Average Recorded Investment | 486 | 191 |
With a specific allowance recorded, Interest Income Recognized | 24 |  |
With a specific allowance recorded, Interest Income Recognized Cash Basis | 2 |  |
Recorded Investment | 625 | 501 |
Unpaid Principal Balance | 625 | 501 |
Related Allowance | 46 | 66 |
Average Recorded Investment | 537 | 618 |
Interest Income Recognized | 25 | 24 |
Interest Income Recognized Cash Basis | 2 | 23 |
Commercial and Industrial [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
With no related allowance recorded, Recorded Investment | 333 |  |
With no related allowance recorded, Unpaid Principal Balance | 333 |  |
With no related allowance recorded, Average Recorded Investment | 85 | 337 |
With no related allowance recorded, Interest Income Recognized | 2 |  |
With a specific allowance recorded, Recorded Investment | 111 | 116 |
With a specific allowance recorded, Unpaid Principal Balance | 111 | 116 |
Related Allowance | 39 | 35 |
With a specific allowance recorded, Average Recorded Investment | 114 | 246 |
With a specific allowance recorded, Interest Income Recognized |  | 24 |
Recorded Investment | 444 | 116 |
Unpaid Principal Balance | 444 | 116 |
Related Allowance | 39 | 35 |
Average Recorded Investment | 199 | 583 |
Interest Income Recognized | $ 2 | $ 24 |

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