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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, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | $ 1,841 | $ 1,131 | $ 1,294 | $ 1,841 | $ 1,294 | $ 1,131
With a specific allowance recorded, Recorded Investment | 970 | 614 | 685 | 970 | 685 | 614
Related Allowance | 148 | 106 | 123 | 148 | 123 | 106
Recorded Investment | 2,811 | 1,745 | 1,979 | 2,811 | 1,979 | 1,745
Average Recorded Investment | 2,158 | $ 2,160 | 1,804 | 1,765 | 1,793 | $ 1,885
Consumer Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 589 |  | 928 | 589 | 928 |
With no related allowance recorded, Unpaid Principal Balance | 589 |  | 928 | 589 | 928 |
With no related allowance recorded, Average Recorded Investment | 645 |  | 926 | 554 | 959 |
With no related allowance recorded, Interest Income Recognized | 9 |  | 8 | 23 | 25 |
With no related allowance recorded, Interest Income Recognized Cash Basis | 5 |  | 5 | 15 | 17 |
With a specific allowance recorded, Recorded Investment | 174 |  | 84 | 174 | 84 |
With a specific allowance recorded, Unpaid Principal Balance | 174 |  | 84 | 174 | 84 |
Related Allowance | 26 |  | 25 | 26 | 25 |
With a specific allowance recorded, Average Recorded Investment | 227 |  | 85 | 153 | 89 |
Recorded Investment | 763 |  | 1,012 | 763 | 1,012 |
Unpaid Principal Balance | 763 |  | 1,012 | 763 | 1,012 |
Average Recorded Investment | 872 |  | 1,011 | 707 | 1,048 |
Interest Income Recognized | 9 |  | 8 | 23 | 25 |
Interest Income Recognized Cash Basis | 5 |  | 5 | 15 | 17 |
Agricultural Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment |  |  | 205 |  | 205 |
With no related allowance recorded, Unpaid Principal Balance |  |  | 205 |  | 205 |
With no related allowance recorded, Average Recorded Investment |  |  | 136 | 22 | 119 |
Recorded Investment |  |  | 205 |  | 205 |
Unpaid Principal Balance |  |  | 205 |  | 205 |
Average Recorded Investment |  |  | 136 | 22 | 119 |
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 |
Commercial Real Estate [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 196 |  |  | 196 |  |
With no related allowance recorded, Unpaid Principal Balance | 196 |  |  | 196 |  |
With no related allowance recorded, Average Recorded Investment | 197 |  |  | 199 |  |
With no related allowance recorded, Interest Income Recognized | 3 |  |  | 8 |  |
With a specific allowance recorded, Recorded Investment |  |  | 488 |  | 488 |
With a specific allowance recorded, Unpaid Principal Balance |  |  | 488 |  | 488 |
Related Allowance |  |  | 67 |  | 67 |
With a specific allowance recorded, Average Recorded Investment |  |  | 489 | 186 | 493 |
With a specific allowance recorded, Interest Income Recognized |  |  | 5 |  | 17 |
With a specific allowance recorded, Interest Income Recognized Cash Basis |  |  |  |  | 2 |
Recorded Investment | 196 |  | 488 | 196 | 488 |
Unpaid Principal Balance | 196 |  | 488 | 196 | 488 |
Average Recorded Investment | 197 |  | 489 | 385 | 493 |
Interest Income Recognized | 3 |  | 5 | 8 | 17 |
Interest Income Recognized Cash Basis |  |  |  |  | 2 |
Commercial and Industrial [Member] |  |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |  |
With no related allowance recorded, Recorded Investment | 1,056 |  |  | 1,056 |  |
With no related allowance recorded, Unpaid Principal Balance | 1,056 |  |  | 1,056 |  |
With no related allowance recorded, Average Recorded Investment | 504 |  |  | 238 |  |
With no related allowance recorded, Interest Income Recognized | 12 |  |  | 12 |  |
With a specific allowance recorded, Recorded Investment | 796 |  | 113 | 796 | 113 |
With a specific allowance recorded, Unpaid Principal Balance | 796 |  | 113 | 796 | 113 |
Related Allowance | 122 |  | 31 | 122 | 31 |
With a specific allowance recorded, Average Recorded Investment | 585 |  | 114 | 413 | 115 |
With a specific allowance recorded, Interest Income Recognized | 3 |  |  | 11 |  |
Recorded Investment | 1,852 |  | 113 | 1,852 | 113 |
Unpaid Principal Balance | 1,852 |  | 113 | 1,852 | 113 |
Average Recorded Investment | 1,089 |  | $ 114 | 651 | $ 115 |
Interest Income Recognized | $ 15 |  |  | $ 23 |  |

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