Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) - Power $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 4	$ 9
Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	23	24
Nodal basis	(9)	(10)
Minimum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	4.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	39	46
Nodal basis	0	0
Maximum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	4	4
Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	28	28
Nodal basis	(2)	(2)
Weighted Average | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	4.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 3	$ 8
Derivative Liability, Fair Value, Gross Asset | $	$ (186)	$ (196)