A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Energy measure of increase in quantity of proved reserve of oil and gas resulting from the extension of the proved acreage of previously discovered reservoir or discovery of new field with proved reserve or of new reservoir of proved reserve in old field, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReserveExtensionAndDiscoveryEnergy
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- Definition

Energy measure of proved reserve of oil and gas, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447

+ Details

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- Definition

Energy measure of decrease in quantity of proved reserve of oil and gas resulting from production, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReserveProductionEnergy
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Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Energy measure of increase in quantity of proved reserve of oil and gas resulting from purchase of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservePurchaseOfMineralInPlaceEnergy
Namespace Prefix:	srt_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Energy measure of increase (decrease) in quantity of proved reserve of oil and gas related to revision of a previous estimate resulting from new information, except for an increase in proved acreage, or change in economic factors, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReserveRevisionOfPreviousEstimateEnergy
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Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Definition

Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesExtensionsDiscoveriesAndAdditions
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- Definition

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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesNet
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Period Type:	instant

- Definition

Production of proved reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesProduction
Namespace Prefix:	srt_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

- Definition

Purchase of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesPurchasesOfMineralsInPlace
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Balance Type:	na
Period Type:	duration

- 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/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesRevisionsOfPreviousEstimatesIncreaseDecrease
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- Definition

Energy measure of decrease in quantity of proved reserve of oil and gas resulting from sale of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesSaleOfMineralInPlaceEnergy
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Period Type:	duration

- Definition

Sales of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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

+ Details

Name:	srt_ProvedDevelopedAndUndevelopedReservesSalesOfMineralsInPlace
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- Definition

Energy measure of interests in proved developed reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

+ References

+ Details

Name:	srt_ProvedDevelopedReservesBOE1
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- Definition

Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

+ References

+ Details

Name:	srt_ProvedDevelopedReservesVolume
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Data Type:	num:volumeItemType
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Period Type:	instant

- Definition

Energy measure of proved undeveloped reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

+ References

+ Details

Name:	srt_ProvedUndevelopedReserveBOE1
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Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

- Definition

Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

+ References

+ Details

Name:	srt_ProvedUndevelopedReserveVolume
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Supplemental Oil and Gas Reserve Information - Unaudited - Net Proved, Proved Developed and Proved Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2018 MBbls	Dec. 31, 2018 MBbls MMcf	Dec. 31, 2018 MBoe MBbls	Dec. 31, 2017 MBbls	Dec. 31, 2017 MBbls MMcf	Dec. 31, 2017 MBoe MBbls	Dec. 31, 2016 MBbls	Dec. 31, 2016 MBbls MMcf	Dec. 31, 2016 MBoe MBbls	Dec. 31, 2018 MMcf	Dec. 31, 2018 MBoe	Dec. 31, 2017 MMcf	Dec. 31, 2017 MBoe	Dec. 31, 2016 MMcf	Dec. 31, 2016 MBoe
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Proved reserves (Volume), beginning balance	224,960	523,456		236,593	411,104		184,928	199,828
Proved reserves (Energy), beginning balance			312,204			305,110			218,233
Revisions of previous estimates (Volume)	(17,352)	3,019		(28,323)	54,726		11,713	116,539
Revisions of previous estimates (Energy)			(16,850)			(19,200)			31,136
Extensions, discoveries and other additions (Volume)	30,640	46,309		36,238	89,489		10,790	24,520
Extensions, discoveries and other additions (Energy)			38,358			51,153			14,876
Sales of reserves in place (Volume)	(12,470)	(20,735)		(1,196)	(1,147)		(5,828)	(10,839)
Sales of reserves in place (Energy)			(15,926)			(1,387)			(7,635)
Purchases of reserves in place (Volume)	25,688	43,107		466	1,230		50,164	100,629
Purchases of reserves in place (Energy)			32,873			671			66,936
Production (Volume)	(23,050)	(42,430)		(18,818)	(31,946)		(15,174)	(19,573)
Production (Energy)			(30,122)			(24,143)			(18,436)
Net proved reserves (Volume), closing balance	228,416	552,726		224,960	523,456		236,593	411,104
Net proved reserves (Energy), closing balance			320,537			312,204			305,110
Proved developed reserves (Volume)	144,533	144,533	144,533	150,628	150,628	150,628	152,337	152,337	152,337	339,444		301,101		229,568
Proved developed reserves (Energy)											201,107		200,812		190,598
Proved undeveloped reserve (Volume)	83,883	83,883	83,883	74,332	74,332	74,332	84,256	84,256	84,256	213,282		222,355		181,536
Proved undeveloped reserves (Energy)											119,430		111,392		114,512