Intangible Assets (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	$ 382
Intangible Assets, Accumulated Amortization	277
Intangible Assets, Net	105
Amortization Expense
Actual, 2017	15
Estimated, 2018	11
Estimated, 2019	10
Estimated, 2020	7
Estimated, 2021	5
Estimated, 2022	5
Estimated, Thereafter	67
NUG contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	124
Intangible Assets, Accumulated Amortization	36
Intangible Assets, Net	88
Amortization Expense
Actual, 2017	5
Estimated, 2018	5
Estimated, 2019	5
Estimated, 2020	5
Estimated, 2021	5
Estimated, 2022	5
Estimated, Thereafter	63
OVEC
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	8
Intangible Assets, Accumulated Amortization	3
Intangible Assets, Net	5
Amortization Expense
Actual, 2017	1
Estimated, 2018	0
Estimated, 2019	1
Estimated, 2020	0
Estimated, 2021	0
Estimated, 2022	0
Estimated, Thereafter	4
Coal contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	102
Intangible Assets, Accumulated Amortization	94
Intangible Assets, Net	8
Amortization Expense
Actual, 2017	4
Estimated, 2018	3
Estimated, 2019	3
Estimated, 2020	2
Estimated, 2021	0
Estimated, 2022	0
Estimated, Thereafter	0
FES customer contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	148
Intangible Assets, Accumulated Amortization	144
Intangible Assets, Net	4
Amortization Expense
Actual, 2017	5
Estimated, 2018	3
Estimated, 2019	1
Estimated, 2020	0
Estimated, 2021	0
Estimated, 2022	0
Estimated, Thereafter	$ 0