EMPLOYEE BENEFIT PLANS (Discount rate and assumptions) (Details)	3 Months Ended									6 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nonunion Defined Benefit Pension Plan
Weighted-average assumptions used to determine nonunion benefit obligations
Discount rate (as a percent)	3.10%			3.40%						3.10%			3.10%	3.40%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	3.10%	3.20%	3.40%	2.70%	2.70%	3.00%	3.40%	3.50%	3.00%				3.40%	3.50%	3.20%
Expected return on plan assets, net of estimated expenses (as a percent)										2.50%	6.50%			6.50%	6.50%
Nonunion Defined Benefit Pension Plan | Forecast
Weighted-average assumptions used to determine net periodic benefit cost
Expected return on plan assets, net of estimated expenses (as a percent)												1.40%
Supplemental Benefit Plan
Weighted-average assumptions used to determine nonunion benefit obligations
Discount rate (as a percent)	2.80%			2.70%						2.80%			2.80%	2.70%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)													2.70%	2.60%	2.50%
Postretirement Health Benefit Plan
Weighted-average assumptions used to determine nonunion benefit obligations
Discount rate (as a percent)	3.50%			4.00%						3.50%			3.50%	4.00%
Weighted-average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)													4.00%	4.20%	3.90%