Accumulated Other Comprehensive Income (Schedule Of Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 27, 2021	Mar. 28, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment income (loss) and interest expense	$ 1,611	$ (4,019)
Provision for income taxes	(8,415)	(9,396)
Net income	24,255	26,689
Unrealized Gains (Losses) On Available-For-Sale Marketable Securities [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment income (loss) and interest expense		(8)
Provision for income taxes		2
Net income		$ (6)