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Intangible Assets (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Intangible Assets (Textuals) [Abstract]  
Intangible Assets, Gross $ 234
Intangible Assets, Accumulated Amortization 141
Intangible Assets, Net 93
Amortization Expense  
Actual, 2018 8
Estimated, 2019 9
Estimated, 2020 7
Estimated, 2021 5
Estimated, 2022 5
Estimated, 2023 6
Estimated, Thereafter 61
NUG contracts  
Intangible Assets (Textuals) [Abstract]  
Intangible Assets, Gross 124
Intangible Assets, Accumulated Amortization 41
Intangible Assets, Net 83
Amortization Expense  
Actual, 2018 5
Estimated, 2019 5
Estimated, 2020 5
Estimated, 2021 5
Estimated, 2022 5
Estimated, 2023 5
Estimated, Thereafter 58
OVEC  
Intangible Assets (Textuals) [Abstract]  
Intangible Assets, Gross 8
Intangible Assets, Accumulated Amortization 3
Intangible Assets, Net 5
Amortization Expense  
Actual, 2018 0
Estimated, 2019 1
Estimated, 2020 0
Estimated, 2021 0
Estimated, 2022 0
Estimated, 2023 1
Estimated, Thereafter 3
Coal contracts  
Intangible Assets (Textuals) [Abstract]  
Intangible Assets, Gross 102
Intangible Assets, Accumulated Amortization 97
Intangible Assets, Net 5
Amortization Expense  
Actual, 2018 3
Estimated, 2019 3
Estimated, 2020 2
Estimated, 2021 0
Estimated, 2022 0
Estimated, 2023 0
Estimated, Thereafter $ 0