SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND BASIS OF PRESENTATION - Concentration of Credit Risk (Details) - Revenue - Customer	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Great Western Operating Company
Concentration of credit risk
Concentration risk (as a percent)	22.00%	43.00%
Kerr-McGee Oil and Gas Onshore
Concentration of credit risk
Concentration risk (as a percent)	4.00%	23.00%
Ward Petroleum
Concentration of credit risk
Concentration risk (as a percent)	2.00%	20.00%
DCP Midstream
Concentration of credit risk
Concentration risk (as a percent)	1.00%	11.00%
PDC Energy, Inc.
Concentration of credit risk
Concentration risk (as a percent)	71.00%	0.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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