Investments (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	$ 16,000,000.0		$ 18,500,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, Additional Credit Losses	0	$ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 1,800,000
Percent of average daily trading volume	0.20%	0.20%	0.20%
Carrying Value of Securities Pledged	$ 467,000,000		$ 510,000,000
Non-Governmental Issues Exceeding 10% Shareholders Equity Threshold	0		0
Equity Securities
Schedule of Investments [Line Items]
Equity Securities, FV-NI	$ 12,000,000.0		$ 11,800,000
Maximum | First National Corporation (FXNC)
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	4.00%		4.00%
Maximum | Eagle Financial Services, Inc. (EFSI)
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	1.50%		1.50%