POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medical Benefit Obligations Income Statement Disclosures (Details) - Postretirement Medical Benefits - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 4,288	$ 4,029	$ 12,866	$ 12,090
Former Mining Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	2,034	2,402	6,103	7,210
Current Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 2,254	$ 1,627	$ 6,763	$ 4,880