DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) MMcf	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) MMcf	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative, Nonmonetary Notional Amount | MMcf	35,000		35,000		14,100
Premiums recorded in other current assets on natural gas contracts held	$ 7,600,000		$ 7,600,000		$ 5,500,000
Fair Value Assets, Transfers between Levels	0	$ 0	0	$ 0
Long-term Debt, including current maturities	893,880,000		893,880,000		1,193,257,000
Long-term Debt	1,200,000,000		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	8,600,000		8,600,000		1,100,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term Debt, Fair Value	$ 1,200,000,000		$ 1,200,000,000		$ 1,300,000,000