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Property, Plant and Equipment (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Mar. 31, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Property Plant And Equipment [Line Items]          
Asset impairments       $ 2,666  
Exploration expenses $ 39   $ 18 163 $ 29
Proved Asset Impairments [Member]          
Property Plant And Equipment [Line Items]          
Asset impairments   $ 2,700   2,700  
Unproved Impairments [Member]          
Property Plant And Equipment [Line Items]          
Exploration expenses $ 36   $ 13 $ 149 $ 15