SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 27, 2021	Mar. 28, 2020
Net periodic (benefit) expense:
Interest cost	$ 2,497	$ 3,124
Expected return on plan assets	(7,005)	(5,598)
Amortization of actuarial loss	832	711
Net periodic (benefit) expense	$ (3,676)	$ (1,763)