POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medicial Benefit Obligations Balance Sheet Disclosures (Details) (Postretirement Medical Benefits, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Postretirement Medical Benefits
Components of net periodic benefit cost:
Service cost	$ 1,054	$ 822
Interest cost	2,907	3,203
Amortization of deferred items	327	5
Total net periodic benefit cost	$ 4,288	$ 4,030