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Asset Retirement Obligations (Summary Of Changes In Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations as of beginning of period $ 398 $ 484  
Liabilities incurred 11 8  
Liabilities settled and divested (21) (39)  
Revision of estimated obligation 4 (62)  
Accretion expense on discounted obligation 10 11  
Asset retirement obligations as of end of period 402 402  
Less current portion 11 15  
Asset retirement obligations, long-term $ 391 $ 387 $ 380