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Asset Retirement Obligations (Summary of Changes in Asset Retirement Obligations) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Asset Retirement Obligation Disclosure [Abstract]      
Asset retirement obligations as of beginning of period $ 807 $ 665  
Assumed Grayson Mill obligations 0 75  
Liabilities incurred 50 21  
Liabilities settled and divested (54) (25)  
Revision and reclassification of estimated obligation 55 35  
Accretion expense on discounted obligation 37 28  
Asset retirement obligations as of end of period 895 799  
Less current portion 45 34  
Asset retirement obligations, long-term $ 850 $ 765 $ 770