STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS RELATING TO PROVED OIL, NATURAL GAS AND NATURAL GAS LIQUIDS RESERVES (UNAUDITED) (Schedule of Future Net Cash Flows) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Standardized Measure of Discontinued Future Net Cash Flows Relating to Proved Oil, Natural Gas and Natural Gas Liquids Reserves [Abstract]
Future cash inflows	$ 2,710,234	$ 2,778,662	$ 2,261,520
Future production and development costs	(1,622,040)	(1,603,373)	(1,480,900)
Future income tax expenses	(167,692)	(6,022)	(5,442)
Future net cash flows	920,502	1,169,267	775,178
10% annual discount for estimated timing of cash flows	(484,069)	(589,862)	(404,061)
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total	$ 436,433	$ 579,405	$ 371,117	$ 536,450
Future net cash flows, annual discount rate	10.00%	10.00%	10.00%