Investments (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Schedule of Investments [Line Items]
Percent Of Average Daily Trading Volume	0.20%		0.20%
Credit-related net investment impairment losses	$ 0	$ 0	$ 0
Carrying value of securities pledged	274,000,000		273,000,000
Other Than Temporary Impairment Credit Losses Recognized In Earnings	$ 18,200,000		$ 18,170,000
First National Corporation (FXNC) [Domain] | Maximum [Member]
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	4.00%		4.00%
Eagle Financial Services, Inc. (EFSI) [Domain] | Maximum [Member]
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	1.50%		1.50%
Debt Securities [Member]
Schedule of Investments [Line Items]
Other Than Temporary Impairment Credit Losses Recognized In Earnings	$ 16,600,000		$ 16,586,000
Equity Securities [Member]
Schedule of Investments [Line Items]
Other Than Temporary Impairment Credit Losses Recognized In Earnings	$ 1,600,000		$ 1,584,000