POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medical Benefit Obligations Income Statement Disclosures (Details) (Postretirement Medical Benefits, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 5,145	$ 4,280
Former Mining Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	3,119	2,829
Current Operations
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 2,026	$ 1,451