NATURE OF BUSINESS, SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND OTHER - Concentration of Customers (Details) - Oil And Gas Sales And Other Revenue	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Principal customers, Phillips 66 Company and Valero Marketing & Supply Company
Revenue Recognition
Concentration risk percentage	46.00%	43.00%
Principal customers, Phillips 66 Company, Andeavor, Valero Marketing & Supply Company and Shell Trading (US) Company
Revenue Recognition
Concentration risk percentage	67.00%	67.00%	67.00%
Minimum | Principal customers | Major Customers
Revenue Recognition
Concentration risk percentage	10.00%	10.00%	10.00%
Minimum | Valero Marketing Supply Company | Major Customers
Revenue Recognition
Concentration risk percentage	10.00%	10.00%	10.00%
Minimum | Phillips66 Company | Major Customers
Revenue Recognition
Concentration risk percentage	10.00%	10.00%	10.00%
Minimum | Marathon Petroleum Corporation Formerly Andeavor | Major Customers
Revenue Recognition
Concentration risk percentage		10.00%	10.00%
Minimum | Shell Trading U S Company | Major Customers
Revenue Recognition
Concentration risk percentage		10.00%	10.00%