EMPLOYEE BENEFIT PLANS - Discount rate and assumptions (Details)	3 Months Ended					12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Nonunion Defined Benefit Pension Plan
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.70%	3.80%	3.60%	3.80%	3.60%		3.90%	3.10%
Expected return on plan assets, net of estimated expenses (as a percent)	0.00%	0.00%					1.40%	1.40%
Supplemental Benefit Plan
Weighted-average assumptions used to determine nonunion benefit obligations
Discount rate (as a percent)						1.10%	2.40%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)						2.40%	3.60%	2.80%
Postretirement Health Benefit Plan
Weighted-average assumptions used to determine nonunion benefit obligations
Discount rate (as a percent)						2.30%	3.10%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)						3.10%	4.20%	3.50%