POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medical Benefit Obligations Income Statement Disclosures (Details) - Postretirement Medical Benefits - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 4,751	$ 4,955	$ 9,503	$ 9,909
Former Mining Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	2,046	2,306	4,092	4,611
Current Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 2,705	$ 2,649	$ 5,411	$ 5,298