POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Postretirement Medicial Benefit Obligations Balance Sheet Disclosures (Details) - Postretirement Medical Benefits - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of net periodic benefit cost:
Service cost	$ 889	$ 794	$ 1,779	$ 1,587
Interest cost	2,893	3,196	5,786	6,393
Amortization of deferred items	969	965	1,938	1,929
Total net periodic benefit cost	$ 4,751	$ 4,955	$ 9,503	$ 9,909