Investments - Madison Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 24, 2017	Jun. 25, 2017	Mar. 26, 2017	Dec. 25, 2016	Sep. 25, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015
Income Statement Information [Abstract]
Loss from discontinued operations, net of income taxes	$ 57	$ (488)	$ 0	$ 0	$ (2,273)	$ 0	$ 0	$ 0	$ (431)	$ (2,273)	$ 0
Madison Paper Industries [Member]
Balance Sheet Information [Abstract]
Current assets	35,764				3,766				35,764	3,766
Noncurrent assets	9,640				8,944				9,640	8,944
Total assets	45,404				12,710				45,404	12,710
Current liabilities	137				1,373				137	1,373
Noncurrent liabilities	4,070				29,386				4,070	29,386
Total liabilities	4,207				30,759				4,207	30,759
Total equity	$ 41,197				(18,049)				41,197	(18,049)
Income Statement Information [Abstract]
Revenues									0	40,523	133,319
Cost of sales									(13,396)	(63,439)	(126,292)
General and administrative income/(expense) and other									55,058	(62,759)	(13,550)
Total income/(expense)									41,662	(126,198)	(139,842)
Operating income/(loss)									41,662	(85,675)	(6,523)
Other income/(expense)									18	2	689
Net income/(loss)									41,680	$ (85,673)	$ (5,834)
Madison Paper Industries [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of assets					$ 3,900				20,800
Income Statement Information [Abstract]
Loss from discontinued operations, net of income taxes									$ 11,600