Supplemental Balance Sheet Information Related to Leases (Details) - USD ($) $ in Thousands	Sep. 25, 2020	Dec. 31, 2019
Operating Leases
Operating lease right-of-use assets	$ 62,235	$ 23,413
Other liabilities and accrued items	6,781	6,542
Operating lease liabilities	56,891	18,091
Finance Leases
Finance lease liabilities	20,540	17,424
Total principal payable on finance leases	23,240	18,689
Property, plant, and equipment	987,902	916,965
Allowances for depreciation, depletion, and amortization	(693,223)	(684,689)
Finance lease assets, net	294,679	232,276
Other liabilities and accrued items	2,700	1,265
Property, Plant and Equipment
Property, plant, and equipment	34,500	26,069
Allowances for depreciation, depletion, and amortization	(7,045)	(3,570)
Finance lease assets, net	$ 27,455	$ 22,499