Accumulated Other Comprehensive Income (Schedule Of Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment income (loss) and interest expense	$ 1,890	$ 1,467	$ 937	$ 5,441
Provision for income taxes	(12,142)	(4,652)	(36,516)	(15,696)
Net income	31,336	14,319	99,497	49,098
Unrealized Gains 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		(55)		(55)
Provision for income taxes		16		16
Net income		$ (39)		$ (39)