DERIVATIVE INSTRUMENTS AND PRICE RISK MANAGEMENT - Reconciliation between the gross assets and liabilities and the amounts reflected on the balance sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Offsetting Assets [Line Items]
Current Derivative Assets	$ 18,578	$ 115,870
Noncurrent Derivative Assets	17,839	61,843
Total derivative assets	36,417	177,713
Current liabilities	(5,882)	0
Noncurrent liabilities	(4,991)	0
Total derivative liabilities	(10,873)	0
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	57,508	178,477
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(21,091)	(764)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	36,417	177,713
Gross Amounts of Recognized Assets, Offsetting of Derivative Liabilities	(31,964)	(764)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	21,091	764
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(10,873)	0
Current Assets
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	31,558	116,620
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(12,980)	(750)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	18,578	115,870
Noncurrent Assets
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Assets	25,950	61,857
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Assets	(8,111)	(14)
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Assets	17,839	61,843
Current Liabilities
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Liabilities	(18,862)	(750)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	12,980	750
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(5,882)	0
Noncurrent Liabilities
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets, Offsetting of Derivative Liabilities	(13,102)	(14)
Gross Amounts Offset in the Balance Sheet, Offsetting of Derivative Liabilities	8,111	14
Net Amounts of Assets Presented in the Balance Sheet, Offsetting of Derivative Liabilities	(4,991)	0
Swap Price Contracts | Current Assets
Offsetting Assets [Line Items]
Current Derivative Assets	17,973	108,514
Swap Price Contracts | Noncurrent Assets
Offsetting Assets [Line Items]
Noncurrent Derivative Assets	17,839	61,843
Swap Price Contracts | Current Liabilities
Offsetting Assets [Line Items]
Current liabilities	(5,882)	0
Swap Price Contracts | Noncurrent Liabilities
Offsetting Assets [Line Items]
Noncurrent liabilities	(4,991)	0
Basis Swap Contracts | Current Assets
Offsetting Assets [Line Items]
Current Derivative Assets	$ 606	$ 7,356