CONCENTRATION OF RISK Other accrued expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Risks and Uncertainties [Abstract]
Accrued Employee Benefits	$ 22,383	$ 22,282
Asset Retirement Obligation, Current	15,801	9,775
Accrued Environmental Liabilities	1,374	3,238
Other Accrued Liabilities	3,429	3,330
Other Accrued Liabilities, Noncurrent	$ 42,987	$ 38,625