DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) MMcf	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) MMcf	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Document Period End Date			Jun. 30, 2019
Derivative, Nonmonetary Notional Amount | MMcf	15,100		15,100		14,300
Premiums recorded in other current assets on natural gas contracts held	$ 3,900,000		$ 3,900,000		$ 4,100,000
Fair Value Assets, Transfers between Levels	0	$ 0	0	$ 0
Amount of investments included in other current assets	1,500,000		1,500,000
Amount of investments included in other noncurrent assets	1,500,000		1,500,000
Long-term Debt, including current maturities	1,285,811,000		1,285,811,000		1,285,483,000
Long-term Debt	1,300,000,000	$ 1,300,000,000	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	3,000,000.0		3,000,000.0		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	$ 1,500,000,000		$ 1,500,000,000		$ 1,400,000,000