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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
3 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Asset Retirement Obligation [Abstract]    
Asset retirement obligation $ 630 $ 570
McElroy's Run Impoundment Site | Pleasants Power Station    
Public Utility, Property, Plant and Equipment [Line Items]    
Increase in asset retirement obligation $ 43