Concentrations - Customers from Whom We Derived 10% or More of Total Oil and Gas Revenue (Detail)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Conoco, Inc [Member]
Concentration Risk [Line Items]
Concentration risk, benchmark description	Less than 10%	Less than 10%
Customer Concentration Risk [Member] \| Sales Revenue, Product Line [Member] \| Phillips 66 Company [Member]
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	53.00%	31.00%	35.00%
Customer Concentration Risk [Member] \| Sales Revenue, Product Line [Member] \| Shell Trading (US) Company [Member]
Concentration Risk [Line Items]
Customers from whom 10% or more revenue derived	13.00%	32.00%	33.00%

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=6927468&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=35750535&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=6927468&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=35750535&loc=d3e13537-108611

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Data Type:	num:percentItemType
Balance Type:	na
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