DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) MMcf	Dec. 31, 2017 USD ($) MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative, Nonmonetary Notional Amount | MMcf	14,300	14,100
Premiums recorded in other current assets on natural gas contracts held	$ 4,100,000	$ 5,500,000
Fair Value Assets, Transfers between Levels	0	0
Long-term Debt	1,300,000,000	1,200,000,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value of natural gas call options held	2,100,000	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	$ 1,400,000,000	$ 1,300,000,000