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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Data Type:	num:percentItemType
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Organization and Summary of Significant Accounting Policies - Revenues (Details) - Oil and Gas Revenue - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Superior Natural Gas
Concentration Risk [Line Items]
Net oil and gas revenues (percent)	29.00%	14.00%
Shell Trading Company
Concentration Risk [Line Items]
Net oil and gas revenues (percent)	24.00%	23.00%	18.00%
Shell Trading Company
Concentration Risk [Line Items]
Net oil and gas revenues (percent)		17.00%	21.00%
BG Group
Concentration Risk [Line Items]
Net oil and gas revenues (percent)		10.00%	10.00%
Unimark, LLC
Concentration Risk [Line Items]
Net oil and gas revenues (percent)			17.00%