DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) MMcf	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative, Nonmonetary Notional Amount | MMcf			14,300
Premiums recorded in other current assets on natural gas contracts held			$ 4,100,000
Fair Value Assets, Transfers between Levels	$ 0	$ 0
Amount of corporate bonds included in other current assets	1,500,000
Amount of treasury notes included in other current assets	1,400,000
Long-term Debt, including current maturities	1,285,587,000		1,285,483,000
Long-term Debt	1,300,000,000		1,300,000,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair value, natural gas call options			2,100,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term Debt, Fair Value	$ 0		$ 1,400,000,000	$ 1,400,000,000