Shareholders' Equity, Compliance with Regulatory Capital Requirements (Details) $ in Thousands	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
CET1 capital to risk-weighted assets [Abstract]
Actual Amount	$ 463,607	$ 450,687
Minimum Capital Requirement, Amount	$ 179,354	$ 173,674
CET1 capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1163	0.1168
Minimum Capital Requirement, Ratio	0.0450	0.0450
Tier 1 capital to risk-weighted assets [Abstract]
Actual Amount	$ 473,607	$ 460,687
Minimum Capital Requirement, Amount	$ 239,139	$ 231,566
Tier 1 capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1188	0.1194
Minimum Capital Requirement, Ratio	0.0600	0.0600
Risk-based capital to risk-weighted assets [Abstract]
Actual Amount	$ 523,580	$ 509,091
Minimum Capital Requirement, Amount	$ 318,851	$ 308,755
Risk-based capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1314	0.1319
Minimum Capital Requirement, Ratio	0.0800	0.0800
Tier 1 leverage capital ratio [Abstract]
Actual Amount	$ 473,607	$ 460,687
Minimum Capital Requirement, Amount	$ 224,315	$ 206,606
Tier 1 leverage capital ratio, Ratio [Abstract]
Actual Ratio	0.0845	0.0892
Minimum Capital Requirement, Ratio	0.0400	0.0400
Farmers & Merchants Bank [Member]
CET1 capital to risk-weighted assets [Abstract]
Actual Amount	$ 472,965	$ 459,813
Minimum Capital Requirement, Amount	179,339	173,664
Well Capitalized Requirement, Amount	$ 259,046	$ 250,847
CET1 capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1187	0.1191
Minimum Capital Requirement, Ratio	0.0450	0.0450
Well Capitalized Requirement, Ratio	0.0650	0.0650
Tier 1 capital to risk-weighted assets [Abstract]
Actual Amount	$ 472,965	$ 459,813
Minimum Capital Requirement, Amount	239,119	231,551
Well Capitalized Requirement, Amount	$ 318,826	$ 308,735
Tier 1 capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1187	0.1191
Minimum Capital Requirement, Ratio	0.0600	0.0600
Well Capitalized Requirement, Ratio	0.0800	0.0800
Risk-based capital to risk-weighted assets [Abstract]
Actual Amount	$ 522,934	$ 508,215
Minimum Capital Requirement, Amount	318,826	308,735
Well Capitalized Requirement, Amount	$ 398,532	$ 385,919
Risk-based capital to risk-weighted assets, Ratio [Abstract]
Actual Ratio	0.1312	0.1317
Minimum Capital Requirement, Ratio	0.0800	0.0800
Well Capitalized Requirement, Ratio	0.1000	0.1000
Tier 1 leverage capital ratio [Abstract]
Actual Amount	$ 472,965	$ 459,813
Minimum Capital Requirement, Amount	211,642	206,426
Well Capitalized Requirement, Amount	$ 264,552	$ 258,033
Tier 1 leverage capital ratio, Ratio [Abstract]
Actual Ratio	0.0894	0.0891
Minimum Capital Requirement, Ratio	0.0400	0.0400
Well Capitalized Requirement, Ratio	0.0500	0.0500