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Supplemental Oil and Gas Information (Unaudited) - Principal Sources of Change in the Standardized Measure of Discounted Future Net Cash Flows (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) $ / bbl | Dec. 31, 2019 USD ($) $ / bbl | Dec. 31, 2018 USD ($) $ / bbl
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] |  |  |
Net changes in prices and production costs | $ (5,942.1) | $ (2,993.9) | $ 2,972.6
Net changes in development costs | 2,215.1 | (675.7) | (1,891.1)
Sales and transfers of oil and natural gas produced, net of production costs | (1,123.1) | (2,163.8) | (1,978.6)
Net change due to extensions and discoveries | 568.5 | 1,221.9 | 1,930.3
Net change due to purchases and sales of proved reserves | (14.6) | (628.1) | 3,152.4
Development costs incurred | 736.8 | 1,282.4 | 1,017.3
Accretion of discount | 699.3 | 1,002.0 | 469.5
Revisions of previous quantity estimates | (1,461.3) | (71.2) | (347.8)
Net change in income taxes | 1,112.4 | 574.1 | (967.6)
Net increase (decrease) | (3,209.0) | (2,452.3) | 4,357.0
Standardized measure, beginning balance | 5,827.6 | 8,279.9 | 3,922.9
Standardized measure, ending balance | $ 2,618.6 | $ 5,827.6 | $ 8,279.9
Net crude oil and condensate revenue |  |  |
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] |  |  |
Average production costs per barrel of oil (in dollars per barrel) | $ / bbl | 39.57 | 55.69 | 65.56
Net natural gas revenue |  |  |
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] |  |  |
Average production costs per volume of natural gas (in dollars per cubic foot) | $ / bbl | 1.98 | 2.57 | 3.10

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