Allowance for Loan Losses - Impaired Loans Excluding PCI Loans - Recorded Investment and Unpaid Principal Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	$ 1,048	$ 2,338
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	1,073	2,363
Impaired financing receivables, with an allowance recorded, Recorded Investment	224	328
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	227	332
Impaired financing receivables, with an allowance recorded, Related Allowance	73	107
Impaired financing receivables, Recorded Investment, Total	1,272	2,666
Impaired financing receivables, Unpaid Principal Balance, Total	1,300	2,695
Impaired financing receivables, Related Allowance, Total	73	107
Commercial and Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with an allowance recorded, Recorded Investment		74
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance		74
Impaired financing receivables, with an allowance recorded, Related Allowance		73
Impaired financing receivables, Recorded Investment, Total		74
Impaired financing receivables, Unpaid Principal Balance, Total		74
Impaired financing receivables, Related Allowance, Total		73
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	12	757
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	12	757
Impaired financing receivables, with an allowance recorded, Recorded Investment	193	223
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	193	223
Impaired financing receivables, with an allowance recorded, Related Allowance	55	5
Impaired financing receivables, Recorded Investment, Total	205	980
Impaired financing receivables, Unpaid Principal Balance, Total	205	980
Impaired financing receivables, Related Allowance, Total	55	5
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment		48
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance		48
Impaired financing receivables, Recorded Investment, Total		48
Impaired financing receivables, Unpaid Principal Balance, Total		48
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	514	915
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	539	940
Impaired financing receivables, with an allowance recorded, Recorded Investment	31	31
Impaired financing receivables, with an allowance recorded, Unpaid Principal Balance	34	35
Impaired financing receivables, with an allowance recorded, Related Allowance	18	29
Impaired financing receivables, Recorded Investment, Total	545	946
Impaired financing receivables, Unpaid Principal Balance, Total	573	975
Impaired financing receivables, Related Allowance, Total	18	29
Farm Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Impaired financing receivables, with no related allowance recorded, Recorded Investment	522	618
Impaired financing receivables, with no related allowance recorded, Unpaid Principal Balance	522	618
Impaired financing receivables, Recorded Investment, Total	522	618
Impaired financing receivables, Unpaid Principal Balance, Total	$ 522	$ 618