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