Fair Value Measurements (Details 2) - Level 3 $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MWh $ / MWh	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ 9	$ 3	$ 2
NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	(44)	$ (79)	$ (107)
Model | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	9
Model | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair Value | $	$ (44)
Model | Minimum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	0.20
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)	31.40
Model | Maximum | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	6.10
Model | Maximum | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Power (in dollars per MWH) | MWh	1,214,000
Power, Regional prices (in dollars per unit)	33.60
Model | Weighted Average | FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
RTO auction clearing prices (in dollars per unit)	1.80
Model | Weighted Average | NUG contracts
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Power (in dollars per MWH) | MWh	249,000
Power, Regional prices (in dollars per unit)	32.60