Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 25, 2016	Jun. 27, 2015	Jun. 25, 2016	Jun. 27, 2015
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	$ 33,738	$ 96,634	$ 26,268	$ 109,134
Unrealized gain (loss) on AFS securities	21,791	(705)	32,756	(3,917)
Net realized gain on AFS securities reclassified to investment income, net	(731)	(14,032)	(4,226)	(23,465)
Amortization of actuarial losses reclassified to operating and administrative expenses	0	145	0	290
Net other comprehensive earnings (losses)	21,060	(14,592)	28,530	(27,092)
Balances at End of Period	54,798	82,042	54,798	82,042
AFS Securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	38,765	105,317	31,295	117,962
Unrealized gain (loss) on AFS securities	21,791	(705)	32,756	(3,917)
Net realized gain on AFS securities reclassified to investment income, net	(731)	(14,032)	(4,226)	(23,465)
Net other comprehensive earnings (losses)	21,060	(14,737)	28,530	(27,382)
Balances at End of Period	59,825	90,580	59,825	90,580
Postretirement Benefits
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Balances at Beginning of Period	(5,027)	(8,683)	(5,027)	(8,828)
Amortization of actuarial losses reclassified to operating and administrative expenses	0	145	0	290
Net other comprehensive earnings (losses)	0	145	0	290
Balances at End of Period	$ (5,027)	$ (8,538)	$ (5,027)	$ (8,538)