|**Capitalized Exploratory Well Costs and Undeveloped Leasehold Costs (Tables)**                                                             | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|**Extractive Industries [Abstract]**                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Aging of Capitalized Well Costs                                                                                                             | The following table provides an aging of capitalized exploratory well costs based on the date that drilling commenced:(millions, except number of projects)March 31, 2019December 31, 2018Exploratory Well Costs Capitalized for a Period of One Year or Less$9$6Exploratory Well Costs Capitalized for a Period Greater Than One Year Since Commencement of Drilling350348Capitalized Exploratory Well Costs, End of Period$359$354Number of Projects with Exploratory Well Costs That Have Been Capitalized for a Period Greater Than One Year Since Commencement of Drilling77
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|(millions, except number of projects)                                                                                                       | March 31, 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Capitalized Exploratory Well Costs, 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Rollforward of Undeveloped Lease Costs                                                                                                      | Changes in undeveloped leasehold costs were as follows: (millions)Three Months Ended March 31, 2019Undeveloped Leasehold Costs, Beginning of Period$2,306Additions to Undeveloped Leasehold Costs47Transfers to Proved Properties—Assets Sold(2)Undeveloped Leasehold Costs, End of Period$2,351                                                                                                                                                                                                                                                                                 
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|(millions)                                                                                                                                  | Three Months Ended March 31, 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Undeveloped Leasehold Costs, Beginning of Period                                                                                            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Additions to Undeveloped Leasehold Costs                                                                                                    | 47                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Transfers to Proved Properties                                                                                                              | —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Assets Sold                                                                                                                                 | (2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Undeveloped Leasehold Costs, End of Period                                                                                                  | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

X- DefinitionRollforward Of Undeveloped Lease Costs [Table Text Block]
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duration X- DefinitionTabular disclosure of the amount of exploratory well costs that have been capitalized after the completion of drilling and the number of projects for which those costs relate. Additionally, for exploratory well costs that have been capitalized for periods greater than one year at the most recent balance sheet date, this block of text may be used to disclose an aging of those amounts by year, or by using a range of years, and the number of projects to which those costs relate.
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