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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

X

- Definition

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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