Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	$ 882
Intangible Assets, Accumulated Amortization	762
Intangible Assets, Net	120
Amortization Expense
Actual, 2016	114
Estimated, 2017	11
Estimated, 2018	9
Estimated, 2019	6
Estimated, 2020	5
Estimated, 2021	5
Estimated, Thereafter	72
Impairment charges	92
NUG contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	124
Intangible Assets, Accumulated Amortization	31
Intangible Assets, Net	93
Amortization Expense
Actual, 2016	5
Estimated, 2017	5
Estimated, 2018	5
Estimated, 2019	5
Estimated, 2020	5
Estimated, 2021	5
Estimated, Thereafter	68
OVEC
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	54
Intangible Assets, Accumulated Amortization	48
Intangible Assets, Net	6
Amortization Expense
Actual, 2016	2
Estimated, 2017	1
Estimated, 2018	1
Estimated, 2019	0
Estimated, 2020	0
Estimated, 2021	0
Estimated, Thereafter	4
Coal contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	556
Intangible Assets, Accumulated Amortization	544
Intangible Assets, Net	12
Amortization Expense
Actual, 2016	55
Estimated, 2017	0
Estimated, 2018	0
Estimated, 2019	0
Estimated, 2020	0
Estimated, 2021	0
Estimated, Thereafter	0
FES customer contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	148
Intangible Assets, Accumulated Amortization	139
Intangible Assets, Net	9
Amortization Expense
Actual, 2016	52
Estimated, 2017	5
Estimated, 2018	3
Estimated, 2019	1
Estimated, 2020	0
Estimated, 2021	0
Estimated, Thereafter	0
Loss on termination of contract	37
FES
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Net	9	$ 61
FES | Coal contracts
Intangible Assets (Textuals) [Abstract]
Intangible Assets, Gross	$ 40