Fair Value Measurements (Details 2) - Level 3 $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MWh $ / MWh	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ 3	$ 1	$ (5)
FTRs | FES
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	0	(1)	(6)
NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	(79)	$ (107)	$ (136)
Model | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	3
Model | FTRs | FES
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	0
Model | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ (79)
Model | Minimum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	(4.60)
Model | Minimum | FTRs | FES
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	(4.60)
Model | Minimum | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Power (in dollars per MWH) | MWh	400
Power, Regional prices (in dollars per unit)	30.70
Model | Maximum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	5.40
Model | Maximum | FTRs | FES
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	3.30
Model | Maximum | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Power (in dollars per MWH) | MWh	2,099,000
Power, Regional prices (in dollars per unit)	32.00
Model | Weighted Average | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	0.70
Model | Weighted Average | FTRs | FES
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	0.10
Model | Weighted Average | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Power (in dollars per MWH) | MWh	426,000
Power, Regional prices (in dollars per unit)	30.70