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Asset Retirement Obligations (Summary Of Changes In Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations as of beginning of period $ 623 $ 704  
Liabilities incurred 8 17  
Liabilities settled and divested (40) (58)  
Revision of estimated obligation (63)    
Accretion expense on discounted obligation 15 18  
Asset retirement obligations as of end of period 543 681  
Less current portion 15 18  
Asset retirement obligations, long-term $ 528 $ 663 $ 606