Capital Ratios (Narrative) (Details)		9 Months Ended
		Sep. 30, 2017		Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Risk-based Capital Ratio: Ratio		16.89%
Capital Ratios, Basel III, Capital Conservation Buffer		1.25%		0.625%
Capital Ratios, Basel III, Capital Conservation Buffer, Year Two		1.875%
Capital Ratios, Basel III, Capital Conservation Buffer, Year Three		2.50%
Franklin Financial Service Corporation [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Risk-based Capital Ratio: Adequately Capitalized Minimum: Ratio	[1]	8.00%
Total Risk-based Capital Ratio: Ratio	[1]	16.89%		15.67%
Farmers & Merchants Trust Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Risk-based Capital Ratio: Well Capitalized Minimum: Ratio	[2]	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	[4]	5.00%
Total Risk-based Capital Ratio: Well Capitalized Minimum: Ratio	[1]	10.00%
Capital ratios, capital conservation buffer		8.89%
Total Risk-based Capital Ratio: Adequately Capitalized Minimum: Ratio	[1]	8.00%
Total Risk-based Capital Ratio: Ratio	[1]	16.82%		15.55%

[1] Total risk-based capital / total risk-weighted assets

[2] Common equity Tier 1 capital/ total risk-weighted assets

[3] Tier 1 capital / total risk-weighted assets

[4] Tier 1 capital / average quarterly assets