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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]    
Nuclear decommissioning ARO liabilities $ 282 $ 269
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at the beginning of the period 679  
Revisions in estimates for current obligations (27)  
Additions 9  
Spending for current obligations (27)  
Accretion — Expense 30  
Accretion — Nuclear decommissioning 15  
Balance at the ending of the period $ 679