Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	$ (69,068)	$ 43,158	$ 152,636	$ 23,427
Unrealized (loss) gain on debt securities net of income taxes in 2018. Unrealized gain on debt and equity securities net of income taxes in 2017.	(2,600)	23,624	(26,581)	62,542
Net realized loss (gain) on AFS securities reclassified to investment income	425	(38,834)	447	(58,021)
Amortization of actuarial loss reclassified to operating and administrative expenses	564	0	1,129	0
Net other comprehensive earnings (losses)	(1,611)	(15,210)	(25,005)	4,521
Cumulative Effect on Retained Earnings, Net of Tax			(198,310)
Balances at End of Period	(70,679)	27,948	(70,679)	27,948
AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(54,212)	48,849	168,057	29,118
Unrealized (loss) gain on debt securities net of income taxes in 2018. Unrealized gain on debt and equity securities net of income taxes in 2017.	(2,600)	23,624	(26,581)	62,542
Net realized loss (gain) on AFS securities reclassified to investment income	425	(38,834)	447	(58,021)
Net other comprehensive earnings (losses)	(2,175)	(15,210)	(26,134)	4,521
Balances at End of Period	(56,387)	33,639	(56,387)	33,639
Postretirement Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at Beginning of Period	(14,856)	(5,691)	(15,421)	(5,691)
Amortization of actuarial loss reclassified to operating and administrative expenses	564	0	1,129	0
Net other comprehensive earnings (losses)	564	0	1,129	0
Balances at End of Period	$ (14,292)	$ (5,691)	$ (14,292)	$ (5,691)