POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Pension Benefit Disclosures (Details) - Pension - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 608	$ 506
Interest cost	2,287	1,985
Expected return on plan assets	(3,231)	(2,639)
Amortization of deferred items	1,039	1,110
Total net periodic benefit cost	703	$ 962
Employer contributions during the period	200
Expected employer contributions to pension plans during the remainder of the year	$ 200