Investments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Credit-related net investment impairment losses	$ 0.0	$ 0.5	$ 0.0
Other that temporary impairment credit losses recognized in earnings	1.8
Other than temporary impairment, credit losses recognized in earnings, credit losses on debt securities held	0.2
Carrying value of securities pledged	$ 429.0	$ 337.0
nongovernmentalissuerover10%ofequity	0.00%
Maximum
Schedule of Investments [Line Items]
Regional community bank stocks, percent of average trading volumes	0.40%
Equity Securities
Schedule of Investments [Line Items]
Other that temporary impairment credit losses recognized in earnings	$ 1.6
First National Corporation (FXNC) [Domain] | Maximum
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	4.00%
Eagle Financial Services, Inc. (EFSI) [Domain] | Maximum
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies	1.50%