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OPERATING REVENUES - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales	$ 55,307		$ 104,407
Other	108	$ 190	263	$ 1,023
Total operating revenues	55,415	120,137	104,670	255,735
Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales		119,947		254,712
Other		190		1,023
Total operating revenues		120,137		255,735
Oil
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales	48,756		91,825
Total operating revenues	48,756	108,695	91,825	231,216
Oil \| Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales		108,695		231,216
Natural gas
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales	1,560		3,879
Total operating revenues	1,560	5,946	3,879	12,165
Natural gas \| Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales		5,946		12,165
Natural gas liquids
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales	4,991		8,703
Total operating revenues	4,991	5,306	8,703	11,331
Natural gas liquids \| Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of operating revenue
Total oil, natural gas and natural gas liquids sales		$ 5,306		$ 11,331
Accounts Receivable
Disaggregation of operating revenue
Due from contracts with customers	$ 29,400		$ 29,400		$ 24,100