Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 27, 2021	Mar. 28, 2020
Net Investment Income [Line Items]
Investment income (loss)	$ 844	$ 1,300
Unrealized gain (loss) on equity securities	237	(1,922)
Investment income (loss) and interest expense	1,611	(4,019)
Supplemental Employee Retirement Plan [Member]
Net Investment Income [Line Items]
Investment income (loss) and interest expense	$ 775	$ (2,700)

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).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(d))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=120269885&loc=SL75117546-209714

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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section S99
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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities plus amount of the cost of borrowed funds accounted for as interest expense.

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