Accumulated Other Comprehensive Earnings (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 27, 2021	Mar. 28, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	$ 200,951	$ 81,289
Unrealized loss on debt securities	(103,548)	(137,583)
Net realized gain on debt securities reclassified to investment income	(5,583)	(1,726)
Adjustment to postretirement benefit obligation	1,379	815
Net other comprehensive earnings (losses)	(107,752)	(138,494)
Balances at End of Period	93,199	(57,205)
Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	223,904	98,506
Unrealized loss on debt securities	(103,548)	(137,583)
Net realized gain on debt securities reclassified to investment income	(5,583)	(1,726)
Net other comprehensive earnings (losses)	(109,131)	(139,309)
Balances at End of Period	114,773	(40,803)
Postretirement Benefit obligation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(22,953)	(17,217)
Adjustment to postretirement benefit obligation	1,379	815
Net other comprehensive earnings (losses)	1,379	815
Balances at End of Period	$ (21,574)	$ (16,402)