Supplemental Oil and Gas Reserve Information - Unaudited - Standardized Measure of Discounted Future Cash Flows (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 12,385,040	$ 16,652,405	$ 11,636,126
Future production costs	(5,509,127)	(6,609,097)	(4,458,418)
Future development costs	(1,490,521)	(1,701,672)	(992,271)
Future income tax expense	(188,823)	(968,466)	(580,481)
Future net cash flows	5,196,569	7,373,170	5,604,956
10% annual discount for estimated timing of cash flows	(2,352,200)	(3,322,864)	(2,304,261)
Standardized measure of discounted future net cash flows	$ 2,844,369	$ 4,050,306	$ 3,300,695	$ 2,483,065

Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesCashInflows
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

+ References

+ Details

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesDevelopmentCosts
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

+ References

+ Details

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and gas 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 33
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62479-109447

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 55
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=34713648&loc=d3e63094-109448

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 31
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62455-109447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62476-109447

+ Details

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

+ References

+ Details

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

+ References

+ Details

Name:	srt_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of discounted future accumulated cash inflow relating to proved oil and gas 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 31
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62455-109447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 30
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62424-109447

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 55
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=34713648&loc=d3e63094-109448

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62479-109447

+ Details

Name:	srt_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_OilAndGasExplorationAndProductionIndustriesDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration