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Investments (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Schedule of Investments [Line Items]      
Credit-related net investment impairment losses $ 0.0 $ 0.0 $ 0.0
Carrying value of securities pledged $ 291.0 $ 273.0  
Maximum      
Schedule of Investments [Line Items]      
Regional community bank stocks, percent of average trading volumes 0.20%    
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%