FAIR VALUE MEASUREMENTS (Fair Value and Carrying Value of Debt) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 799,963		$ 709,980
Long-term debt, excluding current maturities, Fair value	821,180		744,405
Senior secured notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying value	667,002	[1]	666,538	[1]
Long-term debt, Fair value	688,219	[1]	700,963	[1]
Long-term Line of Credit	95,000	[2]	0	[2]
Lines of Credit, Fair Value Disclosure	95,000	[2]	0	[2]
Catalyst lease [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying value	37,961	[3]	43,442	[3]
Long-term debt, Fair value	$ 37,961	[3]	$ 43,442	[3]

[1] The estimated fair value, categorized as a Level 2 measurement, was calculated based on the present value of future expected payments utilizing implied current market interest rates based on quoted prices of the senior secured notes.

[2] The estimated fair value approximates carrying value, categorized as a Level 2 measurement, as these borrowings bear interest based upon short-term floating market interest rates.

[3] 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.