POSTRETIREMENT MEDICAL BENEFITS AND PENSION - Pension Benefit Disclosures (Details) - Pension - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 410	$ 378	$ 821	$ 785
Interest cost	2,387	2,630	4,777	5,260
Expected return on plan assets	(3,523)	(3,648)	(7,055)	(7,275)
Settlements	0	269	0	269
Amortization of deferred items	119	583	239	1,177
Total net periodic benefit cost	(607)	$ 212	(1,218)	216
Employer contributions during the period			500	$ 200
Expected employer contributions to pension plans during the remainder of the year	$ 2,200		$ 2,200