Allowances for Loan Losses - Summary of Allowances for Loan Losses by Loan Class (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	$ 2,875	$ 2,794	$ 2,789	$ 2,702
Charge-offs	(2)	0	(49)	(2)
Recoveries	5	0	47	0
Provision	457	44	548	138
Ending balance	3,335	2,838	3,335	2,838
Agriculture Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	98	129	120	152
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	31	(5)	9	(28)
Ending balance	129	124	129	124
Commercial Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	350	292	290	312
Charge-offs	0	0	0	0
Recoveries	1	0	19	0
Provision	107	(1)	149	(21)
Ending balance	458	291	458	291
Commercial Real Estate Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	342	264	314	255
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	21	43	49	52
Ending balance	363	307	363	307
Residential Real Estate Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	1,670	1,640	1,702	1,707
Charge-offs	(2)	0	(49)	0
Recoveries	4	0	28	0
Provision	698	47	689	(20)
Ending balance	2,370	1,687	2,370	1,687
Consumer Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	23	33	35	43
Charge-offs	0	0	0	(2)
Recoveries	0	0	0	0
Provision	(3)	(3)	(15)	(11)
Ending balance	20	30	20	30
Municipal Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	16	17	18	13
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	13	2	11	6
Ending balance	29	19	29	19
Unallocated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Beginning balance	376	419	310	220
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(410)	(39)	(344)	160
Ending balance	$ (34)	$ 380	$ (34)	$ 380