PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Interest cost	$ 8,102	$ 6,673	$ 5,463
Estimated return on plan assets	(13,071)	(6,921)	(6,065)
Settlement loss (gain)	3,371	3,538	0
Other	735	(62)	0
Amortization of net loss	1,503	1,313	1,203
Net cost	640	4,541	601
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Interest cost	36	52	46
Estimated return on plan assets	0	0	0
Settlement loss (gain)	7	7	(3)
Other	0	0	0
Amortization of net loss	0	0	0
Net cost	$ 43	$ 59	$ 43