|**Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs (Tables)**                                                             | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|**Extractive Industries [Abstract]**                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Changes in Capitalized Exploratory Well Costs                                                                                               | Changes in capitalized exploratory well costs are as follows and exclude amounts that were capitalized and subsequently expensed in the same period:(millions)Nine Months Ended September 30, 2018Capitalized Exploratory Well Costs, Beginning of Period$520Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves7Divestitures(1)(168)Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves(1)Capitalized Exploratory Well Costs Charged to Expense—Capitalized Exploratory Well Costs, End of Period$358(1)Represents costs primarily related to Gulf of Mexico assets.
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|(millions)                                                                                                                                  | Nine Months Ended September 30, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Capitalized Exploratory Well Costs, Beginning of Period                                                                                     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Additions to Capitalized Exploratory Well Costs Pending Determination of Proved Reserves                                                    | 7                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Divestitures(1)                                                                                                                             | (168                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
|Reclassified to Proved Oil and Gas Properties Based on Determination of Proved Reserves                                                     | (1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Capitalized Exploratory Well Costs Charged to Expense                                                                                       | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Capitalized Exploratory Well Costs, End of Period                                                                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Aging of Capitalized Well Costs                                                                                                             | The following table provides an aging of capitalized exploratory well costs based on the date that drilling commenced:(millions)September 30, 2018December 31, 2017Exploratory Well Costs Capitalized for a Period of One Year or Less$8$10Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling350510Balance at End of Period$358$520Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling78                                                                                                            
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|(millions)                                                                                                                                  | September 30, 2018                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Exploratory Well Costs Capitalized for a Period of One Year or Less                                                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling                                        | 350                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Balance at End of Period                                                                                                                    | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling | 7                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

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