Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) $ / credit $ / MMBTU $ / MWh $ / MW	Dec. 31, 2017 USD ($) $ / credit $ / MMBTU $ / MWh $ / MW
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.37%
Nodal basis	(1.4)	(1.2)
Credit risk	35.00%	0.37%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.92%
Nodal basis	(0.1)	0.1
Credit risk	35.00%	0.37%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		0.53%
Nodal basis	(1)	(1)
Credit risk	35.00%	0.37%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		26.00%
Nodal basis		(0.5)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		46.00%
Nodal basis		(0.3)
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		37.00%
Nodal basis		(0.4)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 0	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ (6)	$ (4)
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.12%	0.12%
Credit risk	35.00%	0.37%
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.91%	0.72%
Credit risk	35.00%	0.37%
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.49%	0.41%
Credit risk	35.00%	0.37%
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	21.00%	20.00%
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	33.00%	26.00%
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	24.00%	22.00%
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 2	$ 3
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ 0	$ 0
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	(10)	(10)
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	22	24
Estimated auction price | $ / MW	(479)	(65)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	0	0
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	37	46
Estimated auction price | $ / MW	1,608	1,823
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	(2)	(2)
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	31	28
Estimated auction price | $ / MW	58	251
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	3	3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	4	4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	3	3
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	5	5
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	7	7
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	6	6
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 5	$ 8
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ (192)	$ (196)