Supplemental Financial Data - Oil and Gas Producing Activities (unaudited) (Details 5) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of estimate of the current market value of the Company's proved reserves
Future cash inflows	$ 2,212,049	$ 2,878,392
Future production costs	(1,306,608)	(1,538,870)
Future development costs	(77,952)	(76,852)
Future income taxes	(146,951)	(258,277)
Future net cash flows	680,538	1,004,393
10% annual discount	(408,690)	(612,325)
Standardized measure of discounted future net cash flows	$ 271,848	$ 392,068	$ 57,082

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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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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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