Accumulated Other Comprehensive Earnings (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 26, 2021	Jun. 27, 2020	Jun. 26, 2021	Jun. 27, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	$ 93,199	$ (57,205)	$ 200,951	$ 81,289
Unrealized gain (loss) on debt securities	12,977	260,078	(90,571)	122,495
Net realized gain on debt securities reclassified to investment income	(3,333)	(2,195)	(8,916)	(3,921)
Adjustment to postretirement benefit obligation	1,380	814	2,759	1,629
Net other comprehensive earnings (losses)	11,024	258,697	(96,728)	120,203
Balances at End of Period	104,223	201,492	104,223	201,492
Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	114,773	(40,803)	223,904	98,506
Unrealized gain (loss) on debt securities	12,977	260,078	(90,571)	122,495
Net realized gain on debt securities reclassified to investment income	(3,333)	(2,195)	(8,916)	(3,921)
Net other comprehensive earnings (losses)	9,644	257,883	(99,487)	118,574
Balances at End of Period	124,417	217,080	124,417	217,080
Postretirement Benefit obligation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(21,574)	(16,402)	(22,953)	(17,217)
Adjustment to postretirement benefit obligation	1,380	814	2,759	1,629
Net other comprehensive earnings (losses)	1,380	814	2,759	1,629
Balances at End of Period	$ (20,194)	$ (15,588)	$ (20,194)	$ (15,588)