FAIR VALUE MEASUREMENTS (Measured on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 9,645	$ 9,325
Catalyst lease obligations [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	34,688	31,802
Catalyst lease obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations	0	0
Catalyst lease obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations	34,688	31,802
Catalyst lease obligations [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	38,338	52,482
Derivative, Collateral, Right to Reclaim Cash	(38,338)	(52,482)
Derivative Liability	0	0
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	34,368
Derivative Liability		49,960
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	3,970
Derivative Liability		2,522
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	507,498	631,280
Money market funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	507,498	631,280
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	53,022	98,609
Derivative assets, Effect of Counter-party Netting	(38,338)	(52,482)
Derivative assets, Net Carrying Value on Balance Sheet	14,684	46,127
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	29,761
Derivative assets, Net Carrying Value on Balance Sheet		63,810
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	21,346
Derivative assets, Net Carrying Value on Balance Sheet		31,256
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	1,915
Derivative assets, Net Carrying Value on Balance Sheet		3,543
Derivatives included with intermediation agreement obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	365	35,511
Derivative assets, Effect of Counter-party Netting	0	0
Derivative assets, Net Carrying Value on Balance Sheet	365	35,511
Derivatives included with intermediation agreement obligations [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	0
Derivative assets, Net Carrying Value on Balance Sheet		0
Derivatives included with intermediation agreement obligations [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	365
Derivative assets, Net Carrying Value on Balance Sheet		35,511
Derivatives included with intermediation agreement obligations [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