FAIR VALUE MEASUREMENTS (Measured on Recurring Basis) (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Current debt	[1]	$ 0		$ 0		$ 2,400,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3		0	$ 0	0	$ 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined Benefit Plan, Plan Assets, Amount		10,400,000		10,400,000		9,700,000
Fair Value, Measurements, Recurring [Member] | Catalyst lease [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Collateral, Right to Reclaim Cash		0		0		0
Catalyst lease obligations		47,200,000		47,200,000		44,300,000
Fair Value, Measurements, Recurring [Member] | Catalyst lease [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations		0		0		0
Fair Value, Measurements, Recurring [Member] | Catalyst lease [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations						44,300,000
Fair Value, Measurements, Recurring [Member] | Catalyst lease [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations		0		0		0
Fair Value, Measurements, Recurring [Member] | 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		11,300,000		11,300,000		2,900,000
Derivative, Collateral, Right to Reclaim Cash		(11,300,000)		(11,300,000)		(2,900,000)
Derivative Liability		0		0		0
Fair Value, Measurements, Recurring [Member] | 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		9,200,000		9,200,000
Derivative Liability						2,700,000
Fair Value, Measurements, Recurring [Member] | 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		2,100,000		2,100,000
Derivative Liability						200,000
Fair Value, Measurements, Recurring [Member] | 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		0
Derivative Liability						0
Fair Value, Measurements, Recurring [Member] | Inventory Supply Arrangement Obligation [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catalyst lease obligations		47,200,000		47,200,000
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		1,300,000		1,300,000		2,800,000
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		1,300,000		1,300,000		2,800,000
Fair Value, Measurements, Recurring [Member] | 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		0
Fair Value, Measurements, Recurring [Member] | 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		0
Fair Value, Measurements, Recurring [Member] | 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		13,900,000		13,900,000		10,100,000
Derivative assets, Effect of Counter-party Netting		(11,300,000)		(11,300,000)		(2,900,000)
Derivative assets, Net Carrying Value on Balance Sheet		2,600,000		2,600,000		7,200,000
Fair Value, Measurements, Recurring [Member] | 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		8,400,000		8,400,000		1,200,000
Fair Value, Measurements, Recurring [Member] | 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		5,500,000		5,500,000		8,900,000
Fair Value, Measurements, Recurring [Member] | 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		0
Derivative assets, Net Carrying Value on Balance Sheet						0
Fair Value, Measurements, Recurring [Member] | 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		12,700,000		12,700,000		24,100,000
Derivative assets, Effect of Counter-party Netting		0		0		0
Derivative assets, Net Carrying Value on Balance Sheet		12,700,000		12,700,000		24,100,000
Fair Value, Measurements, Recurring [Member] | 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		0		0
Fair Value, Measurements, Recurring [Member] | 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		12,700,000		12,700,000		24,100,000
Fair Value, Measurements, Recurring [Member] | 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		$ 0		$ 0

[1] Catalyst leases are valued using a market approach based upon commodity prices for similar instruments quoted in active markets and are categorized as a Level 2 measurement. The Company has elected the fair value option for accounting for its catalyst lease repurchase obligations as the Company’s liability is directly impacted by the change in fair value of the underlying catalyst.