Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) - Power $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2018 USD ($) $ / MMBTU $ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 16	$ 4
Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing | $ / MWh	22	23
Nodal basis | $ / MWh	(6)	(9)
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU	(1)
Minimum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU		3
Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing | $ / MWh	34	39
Nodal basis | $ / MWh	0	0
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU	0
Maximum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU		4
Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing | $ / MWh	25	28
Nodal basis | $ / MWh	(2)	(2)
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU	0
Weighted Average | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value Assumptions, Estimated Future Prices | $ / MMBTU		3
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 14	$ 3
Derivative Liability | $	$ (225)	$ (186)