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Supplemental Gas Data (unaudited) - Capitalized Exploratory Well Cost Activity (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]      
Balance, Beginning of Period $ 8,984 $ 8,178 $ 6,388
Additions to Capitalized Exploratory Well Costs Pending the Determination of Proved Reserves 28,336 66,409 49,213
Reclassifications to Wells, Facilities and Equipment Based on the Determination of Proved Reserves (28,258) (65,603) (46,614)
Capitalized Exploratory Well Costs Charged to Expense 0 0 (809)
Balance, End of Period $ 9,062 $ 8,984 $ 8,178