Supplementary Oil and Gas Information - (Unaudited) - Net Proved Oil and Natural Gas Reserves (Detail)	12 Months Ended
Dec. 31, 2017 MMcf MBbls	Dec. 31, 2016 MMcf MBbls
Oil [Member]
Reserve Quantities [Line Items]
Reserves, beginning balance \| MBbls	25,191	23,727
Revisions of previous estimates \| MBbls	2,451	2,472
Enhanced oil recovery \| MBbls	0	0
Purchases \| MBbls	0	0
Production \| MBbls	(1,146)	(1,008)
Sales in place \| MBbls	0	0
Reserves, ending balance \| MBbls	26,496	25,191
Natural Gas [Member]
Reserve Quantities [Line Items]
Reserves, beginning balance \| MMcf	10,521	50,103
Revisions of previous estimates \| MMcf	17,011	(36,543)
Enhanced oil recovery \| MMcf	0	0
Purchases \| MMcf	0	0
Production \| MMcf	(2,661)	(3,039)
Sales in place \| MMcf	0	0
Reserves, ending balance \| MMcf	9,354	10,521
Peru (member) \| Oil [Member]
Reserve Quantities [Line Items]
Reserves, beginning balance \| MBbls	25,191	23,727
Revisions of previous estimates \| MBbls	2,451	2,472
Enhanced oil recovery \| MBbls	0	0
Purchases \| MBbls	0	0
Production \| MBbls	(1,146)	(1,008)
Sales in place \| MBbls	0	0
Reserves, ending balance \| MBbls	26,496	25,191
Peru (member) \| Natural Gas [Member]
Reserve Quantities [Line Items]
Reserves, beginning balance \| MMcf	10,521	50,103
Revisions of previous estimates \| MMcf	17,011	(36,543)
Enhanced oil recovery \| MMcf	0	0
Purchases \| MMcf	0	0
Production \| MMcf	(2,661)	(3,039)
Sales in place \| MMcf	0	0
Reserves, ending balance \| MMcf	9,354	10,521

Changes in proved reserve estimates resulting from application of improved recovery techniques.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-10.(a)(2))
-URI http://asc.fasb.org/extlink&oid=82851474&loc=d3e511914-122862

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Production of proved reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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- Definition

Purchase of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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- Definition

Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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- Definition

Sales of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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