Accumulated Other Comprehensive Earnings (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	$ 201,492	$ 62,219	$ 81,289	$ (55,762)
Unrealized gain on debt securities in 2020 and 2019, respectively.	5,456	22,131	127,951	139,782
Net realized gain on debt securities reclassified to investment income in 2020 and 2019, respectively.	(3,678)	(439)	(7,599)	(109)
Adjustment to postretirement benefit obligation	814	0	2,443	0
Net other comprehensive earnings	2,592	21,692	122,795	139,673
Balances at End of Period	204,084	83,911	204,084	83,911
AFS Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	217,080	68,948	98,506	(49,033)
Unrealized gain on debt securities in 2020 and 2019, respectively.	5,456	22,131	127,951	139,782
Net realized gain on debt securities reclassified to investment income in 2020 and 2019, respectively.	(3,678)	(439)	(7,599)	(109)
Net other comprehensive earnings	1,778	21,692	120,352	139,673
Balances at End of Period	218,858	90,640	218,858	90,640
Postretirement Benefits [Member}
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(15,588)	(6,729)	(17,217)	(6,729)
Adjustment to postretirement benefit obligation	814	0	2,443	0
Net other comprehensive earnings	814	0	2,443	0
Balances at End of Period	$ (14,774)	$ (6,729)	$ (14,774)	$ (6,729)