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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 |

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