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Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 26, 2020 | Dec. 28, 2019 | Dec. 29, 2018
Net Investment Income [Line Items] |  |  |
Investment income (loss) | $ 536,000 | $ 4,100 | $ (234)
Unrealized gain (loss) on equity securities | (1,791) | 1,975 | (1,606)
Investment income (loss) and interest expense | 3,817 | 7,054 | (1,454)
Supplemental Employee Retirement Plan [Member] |  |  |
Net Investment Income [Line Items] |  |  |
Investment income (loss) and interest expense | $ 3,300 | $ 3,000 | $ 931

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