Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 30, 2017	Sep. 24, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	$ 27,948	$ 54,798	$ 23,427	$ 26,268
Unrealized gain on AFS securities	49,043	21,263	111,585	54,019
Net realized gain on AFS securities reclassified to investment income	(6,334)	(3,725)	(64,355)	(7,951)
Net other comprehensive earnings (losses)	42,709	17,538	47,230	46,068
Balances at End of Period	70,657	72,336	70,657	72,336
AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	33,639	59,825	29,118	31,295
Unrealized gain on AFS securities	49,043	21,263	111,585	54,019
Net realized gain on AFS securities reclassified to investment income	(6,334)	(3,725)	(64,355)	(7,951)
Net other comprehensive earnings (losses)	42,709	17,538	47,230	46,068
Balances at End of Period	76,348	77,363	76,348	77,363
Postretirement Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(5,691)	(5,027)	(5,691)	(5,027)
Net other comprehensive earnings (losses)	0	0	0	0
Balances at End of Period	$ (5,691)	$ (5,027)	$ (5,691)	$ (5,027)