SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019	Jun. 27, 2020	Jun. 29, 2019
Net periodic (benefit) expense:
Interest cost	$ 3,160	$ 4,182	$ 6,284	$ 8,527
Expected return on plan assets	(5,664)	(4,943)	(11,262)	(10,078)
Amortization of actuarial loss	720	634	1,431	1,292
Net periodic (benefit) expense	$ (1,784)	$ (127)	$ (3,547)	$ (259)