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Investments (Narrative) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Schedule of Investments [Line Items] |  |  |
Marketable Securities | $ 1,206,161,000 | $ 887,632,000 |
nongovernmentalissuerover10%ofequity | 0 | 0 |
Credit-related net investment impairment losses | 0 | 0 | $ 0
Asset Pledged as Collateral [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Debt Securities, Available-for-sale, Restricted | $ 644,000,000 | 518,000,000 |
VISA, Inc. (VISA) [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Investment Owned, Balance, Shares | 86,605 |  |
Debt and Equity Securities, Realized Gain (Loss) | $ 17,800,000 |  |
Investment Owned, Balance, Principal Amount |  | 0 |
Equity Securities |  |  |
Schedule of Investments [Line Items] |  |  |
Marketable Securities | 11,800,000 | 12,600,000 |
Federal Home Loan Bank Certificates and Obligations (FHLB) [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Equity Securities without Readily Determinable Fair Value, Amount | $ 15,500,000 | $ 15,900,000 |
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% |  |

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