FAIR VALUE MEASUREMENTS (Measured on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 9,468	$ 9,593
Inventory Supply Arrangement Obligation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	16,546	7,721
Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative Liability	16,546	7,721
Inventory Supply Arrangement Obligation [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Inventory Supply Arrangement Obligation [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	16,546	7,721
Inventory Supply Arrangement Obligation [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Catalyst lease [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Collateral, Right to Reclaim Cash	0	0
Catalyst lease obligations	59,034	59,048
Catalyst lease [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations	0	0
Catalyst lease [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations	59,034	59,048
Catalyst lease [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations	0	0
Commodity contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	58,487	84,708
Derivative, Collateral, Right to Reclaim Cash	(14,392)	(10,388)
Derivative Liability	44,095	74,320
Commodity contract [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	40,561
Derivative Liability		51,673
Commodity contract [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	17,926
Derivative Liability		33,035
Commodity contract [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0
Derivative Liability		0
Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,897	4,730
Money market funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,897	4,730
Money market funds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Money market funds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Commodity contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	14,392	10,388
Derivative assets, Effect of Counter-party Netting	(14,392)	(10,388)
Derivative assets, Net Carrying Value on Balance Sheet	0	0
Commodity contract [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	13,096
Derivative assets, Net Carrying Value on Balance Sheet		10,031
Commodity contract [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	1,296
Derivative assets, Net Carrying Value on Balance Sheet		357
Commodity contract [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 0
Derivative assets, Net Carrying Value on Balance Sheet		$ 0