Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 24, 2016	Sep. 26, 2015	Sep. 24, 2016	Sep. 26, 2015
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	$ 54,798	$ 82,042	$ 26,268	$ 109,134
Unrealized gain (loss) on AFS securities	21,263	(58,631)	54,019	(62,548)
Net realized gain on AFS securities reclassified to investment income, net	(3,725)	(8,914)	(7,951)	(32,379)
Amortization of actuarial losses reclassified to operating and administrative expenses	0	145	0	435
Net other comprehensive earnings (losses)	17,538	(67,400)	46,068	(94,492)
Balances at End of Period	72,336	14,642	72,336	14,642
AFS Securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	59,825	90,580	31,295	117,962
Unrealized gain (loss) on AFS securities	21,263	(58,631)	54,019	(62,548)
Net realized gain on AFS securities reclassified to investment income, net	(3,725)	(8,914)	(7,951)	(32,379)
Net other comprehensive earnings (losses)	17,538	(67,545)	46,068	(94,927)
Balances at End of Period	77,363	23,035	77,363	23,035
Postretirement Benefits
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	(5,027)	(8,538)	(5,027)	(8,828)
Amortization of actuarial losses reclassified to operating and administrative expenses	0	145	0	435
Net other comprehensive earnings (losses)	0	145	0	435
Balances at End of Period	$ (5,027)	$ (8,393)	$ (5,027)	$ (8,393)