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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Components of net periodic benefit cost:
Service cost	$ 763	$ 1,054	$ 2,507	$ 3,163
Interest cost	3,075	2,907	9,278	8,722
Amortization of deferred items	368	327	891	981
Total net periodic benefit cost	$ 4,206	$ 4,288	$ 12,676	$ 12,866