Supplemental Oil and Gas Reserve Information - Unaudited - Standardized Measure of Discounted Future Cash Flows (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 5,197,220	$ 12,385,040	$ 16,652,405
Future production costs	(2,792,921)	(5,509,127)	(6,609,097)
Future development costs	(610,658)	(1,490,521)	(1,701,672)
Future income tax expense	(232,849)	(188,823)	(968,466)
Future net cash flows	1,560,792	5,196,569	7,373,170
10% annual discount for estimated timing of cash flows	(611,915)	(2,352,200)	(3,322,864)
Standardized measure of discounted future net cash flows	$ 948,877	$ 2,844,369	$ 4,050,306	$ 3,300,695

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

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

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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.

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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.

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

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-SubTopic 235
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Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

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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.

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Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

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

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-Name Accounting Standards Codification
-Topic 932
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