DERIVATIVE FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($) MMcf	Jun. 30, 2020 USD ($) MMcf	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) MMcf
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Document Period End Date		Jun. 30, 2020
Derivative, Nonmonetary Notional Amount | MMcf	14,700	14,700		14,300
Premiums recorded in other current assets on natural gas contracts held	$ 5,800,000	$ 5,800,000		$ 4,400,000
Fair Value Assets, Transfers between Levels	0	0	$ 0
Corporate bonds in other assets	2,900,000	2,900,000		2,600,000
Treasury notes in other assets	2,900,000	2,900,000		3,000,000.0
Long-term Debt, including current maturities	1,581,931,000	1,581,931,000		1,286,064,000
Long-term Debt	1,600,000,000	1,600,000,000		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	4,900,000	4,900,000		300,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term Debt, Fair Value	$ 1,900,000,000	$ 1,900,000,000		$ 1,500,000,000