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Asset Retirement Obligations (Summary of Changes in Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations as of beginning of period $ 665 $ 529  
Assumed Grayson Mill obligations 75 0  
Liabilities incurred 21 104  
Liabilities settled and divested (25) (24)  
Revision of estimated obligation 35 27  
Accretion expense on discounted obligation 28 21  
Asset retirement obligations as of end of period 799 657  
Less current portion 34 16  
Asset retirement obligations, long-term $ 765 $ 641 $ 643