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Long-Term Investments (Equity Method Investments) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Long-Term Investments [Line Items]    
Long-Term Investments $ 932 $ 1,050
Keystone [Member]    
Long-Term Investments [Line Items]    
Location of the affiliated companies, equity method investments PA  
Owned percentage 23.00%  
Conemaugh [Member]    
Long-Term Investments [Line Items]    
Location of the affiliated companies, equity method investments PA  
Owned percentage 23.00%  
PennEast [Member]    
Long-Term Investments [Line Items]    
Location of the affiliated companies, equity method investments PA  
Owned percentage 10.00%  
Kalaeloa [Member]    
Long-Term Investments [Line Items]    
Location of the affiliated companies, equity method investments HI  
Owned percentage 50.00%  
Power [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments $ 87 102
Power [Member] | Partnerships And Corporate Joint Ventures [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments [1] 87 102
Power [Member] | Partnerships And Corporate Joint Ventures [Member] | Keystone [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments 8 7
Power [Member] | Partnerships And Corporate Joint Ventures [Member] | Conemaugh [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments 8 8
Power [Member] | Partnerships And Corporate Joint Ventures [Member] | PennEast [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments 0 11
Power [Member] | Partnerships And Corporate Joint Ventures [Member] | Kalaeloa [Member]    
Long-Term Investments [Line Items]    
Long-Term Investments $ 71 $ 76
[1] During the three years ended December 31, 2017, 2016 and 2015, dividends from these investments were $18 million, $18 million and $16 million, respectively.