EMPLOYEE BENEFIT PLANS - Funded status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Nonunion Defined Benefit Pension Plan
Change in benefit obligations
Benefit obligations at beginning of period		$ 33,373
Interest cost		624	$ 4,269
Actuarial (gain) loss		300
Benefits paid		(34,297)
Benefit obligations at end of period			33,373
Change in plan assets
Fair value of plan assets at beginning of period		26,646
Actual return on plan assets		(59)
Employer contributions		7,710
Benefits paid		(34,297)
Fair value of plan assets at end of period			26,646
Supplemental Benefit Plan
Change in benefit obligations
Benefit obligations at beginning of period	$ 3,236	3,948
Interest cost	9	39	108
Actuarial (gain) loss	34	186
Benefits paid	(2,887)	(937)
Benefit obligations at end of period	392	3,236	3,948
Change in plan assets
Employer contributions	2,887	937
Benefits paid	(2,887)	(937)
Funded status at period end	(392)	(3,236)
Accumulated benefit obligation	392	3,236
Postretirement Health Benefit Plan
Change in benefit obligations
Benefit obligations at beginning of period	20,630	29,488
Service cost	187	320	366
Interest cost	576	1,212	837
Actuarial (gain) loss	(2,027)	(9,542)
Benefits paid	(615)	(848)
Benefit obligations at end of period	18,751	20,630	$ 29,488
Change in plan assets
Employer contributions	615	848
Benefits paid	(615)	(848)
Funded status at period end	(18,751)	(20,630)
Accumulated benefit obligation	$ 18,751	$ 20,630