Allowances for Loan Losses - Summary of Portfolio of Nonaccrual Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	$ 416	$ 469
Agriculture Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	0	0
Commercial Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	46	48
Commercial Real Estate Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	0	0
Residential Real Estate Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	370	421
Consumer Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	0	0
Municipal Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable, nonaccrual	$ 0	$ 0