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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Supplemental Oil and Gas Information [Abstract] |  |  |  |  |
Net changes in prices and production costs | [1] | $ 2,972.6 | $ 2,428.4 | $ (1,476.1) |
Net changes in development costs | [1] | (1,891.1) | (724.4) | 544.9 |
Sales and transfers of oil and gas produced, net of production costs | [1] | (1,978.6) | (1,576.0) | (1,196.3) |
Net change due to extensions and discoveries | [1] | 1,930.3 | 807.9 | 280.5 |
Net change due to purchases and sales of proved reserves | [1] | 3,152.4 | 85.9 | (583.4) |
Development costs incurred | [1] | 1,017.3 | 802.7 | 479.6 |
Accretion of discount | [1] | 469.5 | 270.9 | 428.1 |
Revisions of previous quantity estimates | [1] | (347.8) | (109.5) | (49.2) |
Net change in income taxes | [1] | (967.6) | (643.0) | 292.8 |
Net increase (decrease) | [1] | 4,357.0 | 1,342.9 | (1,279.1) |
Standardized measure | [1] | $ 8,279.9 | $ 3,922.9 | $ 2,580.0 | $ 3,859.1
[1] | 2018 includes noncontrolling interest in MP GOM.

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