Investments (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2019
Marketable Securities	$ 887,632,000	$ 812,891,000
Credit-related net investment impairment losses	0	0	$ 0
Other that temporary impairment credit losses recognized in earnings	1,600,000
nongovernmentalissuerover10%ofequity	$ 0	0
Maximum
Schedule of Investments [Line Items]
Regional community bank stocks, percent of average trading volumes	0.20%
Equity Securities
Schedule of Investments [Line Items]
Marketable Securities	$ 12,600,000	11,800,000
Federal Home Loan Bank Certificates and Obligations (FHLB) [Member]
Schedule of Investments [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 15,900,000	18,500,000
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%
Asset Pledged as Collateral [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Restricted	$ 508,000,000	$ 510,000,000