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Loans - Loans Individually Evaluated for Impairment by Portfolio Class of Loans (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | $ 1,294 | $ 1,141 | $ 515 | $ 1,294 | $ 515 | $ 1,141
With a specific allowance recorded, Recorded Investment | 685 | 711 | 891 | 685 | 891 | 711
Related Allowance | 123 | 135 | 125 | 123 | 125 | 135
Recorded Investment | 1,979 | 1,852 | 1,406 | 1,979 | 1,406 | 1,852
Related Allowance | 123 | 135 | 125 | 123 | 125 | 135
Average Recorded Investment | 1,804 | $ 1,684 | 1,499 | 1,793 | 1,843 | $ 1,802
Agricultural Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 205 |  | 132 | 205 | 132 |
With no related allowance recorded, Unpaid Principal Balance | 205 |  | 132 | 205 | 132 |
With no related allowance recorded, Average Recorded Investment | 136 |  | 132 | 119 | 152 |
With no related allowance recorded, Interest Income Recognized |  |  |  |  | 1 |
With a specific allowance recorded, Average Recorded Investment |  |  |  |  | 13 |
Recorded Investment | 205 |  | 132 | 205 | 132 |
Unpaid Principal Balance | 205 |  | 132 | 205 | 132 |
Average Recorded Investment | 136 |  | 132 | 119 | 165 |
Interest Income Recognized |  |  |  |  | 1 |
Agricultural [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 161 |  |  | 161 |  |
With no related allowance recorded, Unpaid Principal Balance | 161 |  |  | 161 |  |
With no related allowance recorded, Average Recorded Investment | 54 |  |  | 18 |  |
Recorded Investment | 161 |  |  | 161 |  |
Unpaid Principal Balance | 161 |  |  | 161 |  |
Average Recorded Investment | 54 |  |  | 18 |  |
Consumer Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 928 |  | 383 | 928 | 383 |
With no related allowance recorded, Unpaid Principal Balance | 928 |  | 383 | 928 | 383 |
With no related allowance recorded, Average Recorded Investment | 926 |  | 51 | 959 | 78 |
With no related allowance recorded, Interest Income Recognized | 8 |  |  | 25 | 1 |
With no related allowance recorded, Interest Income Recognized Cash Basis | 5 |  |  | 17 | 1 |
With a specific allowance recorded, Recorded Investment | 84 |  | 329 | 84 | 329 |
With a specific allowance recorded, Unpaid Principal Balance | 84 |  | 329 | 84 | 329 |
Related Allowance | 25 |  | 35 | 25 | 35 |
With a specific allowance recorded, Average Recorded Investment | 85 |  | 377 | 89 | 387 |
With a specific allowance recorded, Interest Income Recognized |  |  | 4 |  | 11 |
With a specific allowance recorded, Interest Income Recognized Cash Basis |  |  | 1 |  | 7 |
Recorded Investment | 1,012 |  | 712 | 1,012 | 712 |
Unpaid Principal Balance | 1,012 |  | 712 | 1,012 | 712 |
Related Allowance | 25 |  | 35 | 25 | 35 |
Average Recorded Investment | 1,011 |  | 428 | 1,048 | 465 |
Interest Income Recognized | 8 |  | 4 | 25 | 12 |
Interest Income Recognized Cash Basis | 5 |  | 1 | 17 | 8 |
Commercial Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Average Recorded Investment |  |  | 345 |  | 367 |
With no related allowance recorded, Interest Income Recognized |  |  | 4 |  | 17 |
With no related allowance recorded, Interest Income Recognized Cash Basis |  |  | 4 |  | 16 |
With a specific allowance recorded, Recorded Investment | 488 |  | 444 | 488 | 444 |
With a specific allowance recorded, Unpaid Principal Balance | 488 |  | 444 | 488 | 444 |
Related Allowance | 67 |  | 68 | 67 | 68 |
With a specific allowance recorded, Average Recorded Investment | 489 |  | 30 | 493 | 254 |
With a specific allowance recorded, Interest Income Recognized | 5 |  |  | 17 |  |
With a specific allowance recorded, Interest Income Recognized Cash Basis |  |  |  | 2 |  |
Recorded Investment | 488 |  | 444 | 488 | 444 |
Unpaid Principal Balance | 488 |  | 444 | 488 | 444 |
Related Allowance | 67 |  | 68 | 67 | 68 |
Average Recorded Investment | 489 |  | 375 | 493 | 621 |
Interest Income Recognized | 5 |  | 4 | 17 | 17 |
Interest Income Recognized Cash Basis |  |  | 4 | 2 | 16 |
Commercial and Industrial [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Average Recorded Investment |  |  | 446 |  | 450 |
With no related allowance recorded, Interest Income Recognized |  |  | 6 |  | 18 |
With a specific allowance recorded, Recorded Investment | 113 |  | 118 | 113 | 118 |
With a specific allowance recorded, Unpaid Principal Balance | 113 |  | 118 | 113 | 118 |
Related Allowance | 31 |  | 22 | 31 | 22 |
With a specific allowance recorded, Average Recorded Investment | 114 |  | 118 | 115 | 142 |
Recorded Investment | 113 |  | 118 | 113 | 118 |
Unpaid Principal Balance | 113 |  | 118 | 113 | 118 |
Related Allowance | 31 |  | 22 | 31 | 22 |
Average Recorded Investment | $ 114 |  | 564 | $ 115 | 592 |
Interest Income Recognized |  |  | $ 6 |  | $ 18 |

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