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Supplemental Oil and Gas Information (Principal Sources of Change in the Standardized Measure of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Millions | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Supplemental Oil and Gas Information [Abstract] |  |  |  |
Net changes in prices and production costs | $ 2,428.4 | $ (1,476.1) | $ (11,365.5) |
Net changes in development costs | (724.4) | 544.9 | 591.4 |
Sales and transfers of oil and gas produced, net of production costs | (1,576.0) | (1,196.3) | (1,876.9) |
Net change due to extensions and discoveries | 807.9 | 280.5 | 1,145.8 |
Net change due to purchases and sales of proved reserves | 85.9 | (583.4) | (287.4) |
Development costs incurred | 802.7 | 479.6 | 1,725.4 |
Accretion of discount | 270.9 | 428.1 | 1,289.5 |
Revisions of previous quantity estimates | (109.5) | (49.2) | 163.3 |
Net change in income taxes | (643.0) | 292.8 | 2,568.3 |
Net increase (decrease) | 1,342.9 | (1,279.1) | (6,046.1) |
Standardized measure | $ 3,922.9 | $ 2,580.0 | $ 3,859.1 | $ 9,905.2

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