DERIVATIVE INSTRUMENTS AND PRICE RISK MANAGEMENT - Classification of Outstanding Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Offsetting Assets [Line Items]
Current Derivative Assets	$ 51,290	$ 5,628
Noncurrent Derivative Assets	111	8,554
Current liabilities	(3,078)	(11,298)
Noncurrent liabilities	(14,659)	(8,079)
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	56,218	36,233
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(4,817)	(22,051)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	51,401	14,182
Gross Amounts of Recognized Assets (Liabilities), Offsetting of Derivative Liabilities	(22,554)	(41,428)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	4,817	22,051
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(17,737)	(19,377)
Current Assets
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	52,739	20,164
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(1,449)	(14,536)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	51,290	5,628
Noncurrent Assets
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	3,479	16,069
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(3,369)	(7,515)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	111	8,554
Current Liabilities
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities), Offsetting of Derivative Liabilities	(4,527)	(25,834)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	1,449	14,536
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(3,078)	(11,298)
Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets (Liabilities), Offsetting of Derivative Liabilities	(18,028)	(15,594)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	3,369	7,515
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(14,659)	(8,079)
Commodity Derivative
Offsetting Assets [Line Items]
Total derivative assets	56,218	36,233
Total derivative liabilities	(22,554)	(41,428)
Commodity Basis Swap Contracts | Current Assets
Offsetting Assets [Line Items]
Current Derivative Assets		0
Commodity Basis Swap Contracts | Current Liabilities
Offsetting Assets [Line Items]
Current liabilities	(519)	0
Commodity Price Swaptions Contracts | Noncurrent Liabilities
Offsetting Assets [Line Items]
Noncurrent liabilities	(17,184)	(10,321)
Commodity Price Swap Contracts | Current Assets
Offsetting Assets [Line Items]
Current Derivative Assets	52,702	20,164
Commodity Price Swap Contracts | Noncurrent Assets
Offsetting Assets [Line Items]
Noncurrent Derivative Assets	3,479	16,069
Commodity Price Swap Contracts | Current Liabilities
Offsetting Assets [Line Items]
Current liabilities	(3,434)	(25,834)
Commodity Price Swap Contracts | Noncurrent Liabilities
Offsetting Assets [Line Items]
Noncurrent liabilities	(399)	(5,273)
Interest Rate Swap Contracts | Current Liabilities
Offsetting Assets [Line Items]
Current liabilities	(574)	0
Interest Rate Swap Contracts | Noncurrent Liabilities
Offsetting Assets [Line Items]
Noncurrent liabilities	$ (445)	$ 0