Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / MMBTU $ / credit $ / MW $ / MWh	Dec. 31, 2016 USD ($) $ / MMBTU $ / credit $ / MW $ / MWh $ / lb
Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 7	$ 3
Derivative liabilities		(5)
Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	15
Derivative liabilities	(30)	(14)
Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	9	9
Derivative liabilities	$ (196)	(188)
Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (4)
Discounted Cash Flow | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		(2.00%)
Counterparty credit risk	12.00%	0.13%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	37.00%	0.13%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	(1.2)	(0.80)
Discounted Cash Flow | Minimum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	24	26
Estimated auction price for FTRs | $ / MW	(65)	(71)
Nodal basis | $ / MWh	(10)	(6)
Discounted Cash Flow | Minimum | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		22
Discounted Cash Flow | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		2.00%
Counterparty credit risk	72.00%	0.22%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	92.00%	8.00%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	0.1	0
Discounted Cash Flow | Maximum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	46	44
Estimated auction price for FTRs | $ / MW	1,823	5,270
Nodal basis | $ / MWh	0	0
Discounted Cash Flow | Maximum | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		24
Discounted Cash Flow | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		0.00%
Counterparty credit risk	41.00%	0.15%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	53.00%	1.00%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	(1)	(0.50)
Discounted Cash Flow | Weighted Average | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	28	29
Estimated auction price for FTRs | $ / MW	251	125
Nodal basis | $ / MWh	(2)	(2)
Discounted Cash Flow | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		22
Option Model | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	20.00%	24.00%
Option Model | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	26.00%	31.00%
Nodal basis | $ / MWh	(0.5)	(0.40)
Option Model | Minimum | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Option Model | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	26.00%	66.00%
Option Model | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	46.00%	66.00%
Nodal basis | $ / MWh	(0.3)	(0.10)
Option Model | Maximum | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Option Model | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	22.00%	28.00%
Option Model | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	37.00%	36.00%
Nodal basis | $ / MWh	(0.4)	(0.20)
Option Model | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Fundamental Energy Production Model | Minimum | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Fundamental Energy Production Model | Maximum | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	4	4
Fundamental Energy Production Model | Weighted Average | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Contract Price Allocation | Minimum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	5	5
Contract Price Allocation | Maximum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	7	7
Contract Price Allocation | Weighted Average | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	6	6
Derivative liabilities | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ 0	$ 0
Derivative liabilities | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(4)	(1)
Derivative liabilities | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(196)	(187)
Derivative liabilities | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(4)
Derivative assets | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3
Derivative assets | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1
Derivative assets | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 8
Derivative assets | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0