POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Pension Benefit Disclosures (Details) - Pension - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 407	$ 608
Interest cost	2,630	2,287
Expected return on plan assets	(3,627)	(3,231)
Amortization of deferred items	594	573
Total net periodic benefit cost	4	237
Employer contributions during the period	100	$ 200
Expected employer contributions to pension plans during the remainder of the year	$ 1,100