DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	3 Months Ended
	Mar. 31, 2016 USD ($) MMcf	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative, Nonmonetary Notional Amount | MMcf	0		17,000
Premiums recorded in other current assets on natural gas contracts held			$ 5,800,000
Fair Value Assets, Transfers between Levels	$ 0	$ 0
Long-term Debt, including current maturities	1,191,854,000		1,191,660,000
Long-term Debt	1,200,000,000		1,200,000,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value, natural gas call options			400,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term Debt, Fair Value	$ 1,300,000,000		$ 1,200,000,000