POSTRETIREMENT MEDICAL BENEFITS - Postretirement Medical Benefit Obligations Income Statement Disclosures (Details) - Postretirement Medical Benefits [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts that will be amortized from accumulated other comprehensive loss into net periodic pension cost:
Net actuarial loss	$ 4,493
Prior service credit	(636)
Components of net periodic benefit cost:
Service cost	3,270	$ 4,217	$ 3,289
Interest cost	12,353	11,629	12,814
Amortization of:
Prior service cost (credit)	(636)	(636)	(635)
Actuarial loss	1,895	1,944	653
Total net periodic benefit cost	16,882	17,154	16,121
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	16,882	17,154	16,121
Former Mining Operations [Member]
Amortization of:
Total net periodic benefit cost	8,540	8,137	9,614
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	8,540	8,137	9,614
Current Operations [Member]
Amortization of:
Total net periodic benefit cost	8,342	9,017	6,507
Net periodic postretirement medical benefit costs relating to current and former mining operations:
Total net periodic benefit cost	$ 8,342	$ 9,017	$ 6,507