DERIVATIVE INSTRUMENTS AND PRICE RISK MANAGEMENT - Classification of outstanding financial instruments (Details) - Not Designated as Hedging Instrument - Cash Flow Hedging - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Total Derivative Assets	$ 36,233	$ 177,713
Total Derivative Liabilities	(41,428)	0
Price Swap Contracts | Current Assets
Derivatives, Fair Value [Line Items]
Total Derivative Assets	20,164	108,514
Price Swap Contracts | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Total Derivative Assets	16,069	61,843
Price Swap Contracts | Current Liabilities
Derivatives, Fair Value [Line Items]
Total Derivative Liabilities	(25,834)	0
Price Swap Contracts | Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Total Derivative Liabilities	(5,273)	0
Basis Swap Contracts | Current Assets
Derivatives, Fair Value [Line Items]
Total Derivative Assets	0	7,356
Swaption Price Contracts | Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Total Derivative Liabilities	$ (10,321)	$ 0