POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medicial Benefit Obligations Balance Sheet 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
Components of net periodic benefit cost:
Service cost	$ 1,054	$ 822	$ 3,163	$ 2,467
Interest cost	2,907	3,203	8,722	9,610
Amortization of deferred items	327	4	981	13
Total net periodic benefit cost	$ 4,288	$ 4,029	$ 12,866	$ 12,090