EMPLOYEE BENEFIT PLANS (Funded status) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nonunion Defined Benefit Pension Plan
Change in benefit obligations
Benefit obligations at beginning of period		$ 33,373	$ 137,417
Interest cost		624	4,269	$ 4,514
Actuarial (gain) loss		300	(3,685)
Benefits paid		(34,297)	(105,522)
Settlement loss			894
Benefit obligations at end of period			33,373	137,417
Change in plan assets
Fair value of plan assets at beginning of period		26,646	124,831
Actual return on plan assets		(59)	1,837
Employer contributions	$ 7,700	7,710	5,500
Benefits paid		(34,297)	(105,522)
Fair value of plan assets at end of period			26,646	124,831
Funded status at period end			(6,727)
Accumulated benefit obligation			33,373
Supplemental Benefit Plan
Change in benefit obligations
Benefit obligations at beginning of period		3,948	3,897
Interest cost		39	108	102
Actuarial (gain) loss		186	(57)
Benefits paid		(937)
Benefit obligations at end of period		3,236	3,948	3,897
Change in plan assets
Employer contributions		937
Benefits paid		(937)
Funded status at period end		(3,236)	(3,948)
Accumulated benefit obligation		3,236	3,948
Postretirement Health Benefit Plan
Change in benefit obligations
Benefit obligations at beginning of period		29,488	24,097
Service cost		320	366	489
Interest cost		1,212	837	1,060
Actuarial (gain) loss		(9,542)	4,957
Benefits paid		(848)	(769)
Benefit obligations at end of period		20,630	29,488	$ 24,097
Change in plan assets
Employer contributions		848	769
Benefits paid		(848)	(769)
Funded status at period end		(20,630)	(29,488)
Accumulated benefit obligation		$ 20,630	$ 29,488