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Asset Retirement Obligations (Summary Of Changes In Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations as of beginning of period $ 398 $ 484  
Liabilities incurred 15 14  
Liabilities settled and divested (24) (50)  
Revision of estimated obligation 4 (62)  
Accretion expense on discounted obligation 15 16  
Asset retirement obligations as of end of period 408 402  
Less current portion 10 17  
Asset retirement obligations, long-term $ 398 $ 385 $ 380