Minimum Regulatory Capital Requirements - Summary of Bank's Actual and Required Capital Amounts and Ratios (Detail) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jan. 01, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital to Risk Weighted Assets Actual, Amount	$ 251,649
Total Risk-Based Capital to Risk Weighted Assets Actual, Amount	297,179
Tier 1 Capital to Risk Weighted Assets Actual, Amount	251,649
Tier 1 leverage Capital to Adjusted Total Assets Actual, Amount	$ 251,649
Common Equity Tier 1 Capital to Risk Weighted Assets Actual, Ratio	9.75%
Total Risk-Based Capital to Risk Weighted Assets Actual, Ratio	0.1151
Tier 1 Capital to Risk Weighted Assets Actual, Ratio	0.0975
Tier 1 leverage Capital to Adjusted Total Assets Actual, Ratio	0.0786		0.040
Common Equity Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 116,147
Total Risk-Based Capital to Risk Weighted Assets For Capital Adequacy Purposes, Amount	206,484
Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Amount	154,863
Tier 1 leverage Capital to Adjusted Total Assets For Capital Adequacy Purposes, Amount	$ 128,081
Common Equity Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	4.50%
Total Risk-Based Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	0.0800		0.080
Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	0.0600
Tier 1 leverage Capital to Adjusted Total Assets For Capital Adequacy Purposes, Ratio	0.0400
Common Equity Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	$ 167,768
Total Risk-Based Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	258,105
Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	206,484
Tier 1 leverage Capital to Adjusted Total Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	$ 160,102
Common Equity Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	6.50%
Total Risk-Based Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.1000
Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.0800
Tier 1 leverage Capital to Adjusted Total Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.0500
Farmers & Merchants State Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital to Risk Weighted Assets Actual, Amount	$ 276,467	$ 259,510
Total Risk-Based Capital to Risk Weighted Assets Actual, Amount	301,176	281,085
Tier 1 Capital to Risk Weighted Assets Actual, Amount	276,467	259,510
Tier 1 leverage Capital to Adjusted Total Assets Actual, Amount	$ 276,467	$ 259,510
Common Equity Tier 1 Capital to Risk Weighted Assets Actual, Ratio	10.77%	11.15%
Total Risk-Based Capital to Risk Weighted Assets Actual, Ratio	0.1173	0.1207
Tier 1 Capital to Risk Weighted Assets Actual, Ratio	0.1077	0.1115
Tier 1 leverage Capital to Adjusted Total Assets Actual, Ratio	0.0866	0.0903
Common Equity Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 115,534	$ 104,763
Total Risk-Based Capital to Risk Weighted Assets For Capital Adequacy Purposes, Amount	205,393	186,245
Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Amount	154,045	139,684
Tier 1 leverage Capital to Adjusted Total Assets For Capital Adequacy Purposes, Amount	$ 127,674	$ 114,972
Common Equity Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	4.50%	4.50%
Total Risk-Based Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	0.0800	0.0800
Tier 1 Capital to Risk Weighted Assets For Capital Adequacy Purposes, Ratio	0.0600	0.0600
Tier 1 leverage Capital to Adjusted Total Assets For Capital Adequacy Purposes, Ratio	0.0400	0.0400
Common Equity Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	$ 166,882	$ 151,324
Total Risk-Based Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	256,742	232,806
Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	205,393	186,245
Tier 1 leverage Capital to Adjusted Total Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Amount	$ 159,593	$ 143,714
Common Equity Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Total Risk-Based Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.1000	0.1000
Tier 1 Capital to Risk Weighted Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.0800	0.0800
Tier 1 leverage Capital to Adjusted Total Assets To Be Well Capitalized Under the Prompt Corrective Action Provisions, Ratio	0.0500	0.0500