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Capital Ratios (Narrative) (Details) - USD ($)
$ in Billions
9 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Total Risk-based Capital Ratio: Ratio 15.80%  
Capital Ratios, Basel III, Capital Conservation Buffer, Year Two 1.875%  
Capital Ratios, Basel III, Capital Conservation Buffer, Year Three 2.50%  
Consolidated asset limit on small bank holding companies $ 3  
Farmers & Merchants Trust Company [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier 1 Risk-based Capital Ratio: Ratio [1] 14.27% 13.80%
Tier One Leverage Capital to Average Assets [2] 9.28% 9.68%
Common Equity Tier 1 Risk-based Capital Ratio: Well Capitalized Minimum: Ratio [1] 6.50%  
Tier 1 Risk-based Capital Ratio: Well Capitalized Minimum: Ratio 8.00% [3] 6.00%
Tier 1 Leverage Ratio: Well Capitalized Minimum: Ratio [2] 5.00%  
Total Risk-based Capital Ratio: Well Capitalized Minimum: Ratio [4] 10.00%  
Capital ratios, capital conservation buffer 7.80%  
Total Risk-based Capital Ratio: Ratio [4] 15.52% 15.06%
[1] Common equity Tier 1 capital/ total risk-weighted assets
[2] Tier 1 capital / average quarterly assets
[3] Tier 1 capital / total risk-weighted assets
[4] Total risk-based capital / total risk-weighted assets