EMPLOYEE BENEFIT PLANS - Changes in Pension Plan Level 3 Assets (Details) - Pension Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Summary of changes in fair value of the pension plans' level 3 assets
Fair value of plan assets at January 1	$ 1,978,708	$ 1,725,543
Return on plan assets:
Fair value of plan assets at December 31	2,303,996	1,978,708
Level 3
Summary of changes in fair value of the pension plans' level 3 assets
Fair value of plan assets at January 1	118,218	121,836
Return on plan assets:
Relating to assets still held at the reporting date	8,969	5,752
Relating to assets sold during the period	0	(44)
Purchases, sales, settlements and expenses	(4,047)	(9,326)
Fair value of plan assets at December 31	$ 123,140	$ 118,218