Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions	Jun. 30, 2018 USD ($) $ / MW $ / MMBTU $ / MWh $ / credit	Dec. 31, 2017 USD ($)	Jun. 30, 2017 $ / MW $ / MMBTU $ / MWh $ / credit
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 1
Derivative liabilities | $	(225)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	1	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	5	(4)
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	5	3
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	(1)	0
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	7	8
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ 192	$ (196)
Measurement Input, Counterparty Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.0037
Measurement Input, Counterparty Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.0092
Measurement Input, Counterparty Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.0053
Measurement Input, Counterparty Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0012		0.0012
Measurement Input, Counterparty Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0085		0.0072
Measurement Input, Counterparty Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0038		0.0041
Measurement Input, Counterparty Credit Risk [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0091		0.0028
Measurement Input, Counterparty Credit Risk [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0091		0.0028
Measurement Input, Counterparty Credit Risk [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0091		0.0028
Measurement Input, Entity Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Measurement Input, Entity Credit Risk [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Measurement Input, Entity Credit Risk [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Measurement Input, Entity Credit Risk [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Measurement Input, Commodity Forward Price [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			24
Measurement Input, Commodity Forward Price [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			46
Measurement Input, Commodity Forward Price [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			28
Measurement Input, Price Volatility [Member] | Natural Gas | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.26
Measurement Input, Price Volatility [Member] | Natural Gas | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.46
Measurement Input, Price Volatility [Member] | Natural Gas | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.37
Measurement Input, Price Volatility [Member] | Fuel Oils | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.20		0.20
Measurement Input, Price Volatility [Member] | Fuel Oils | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.34		0.26
Measurement Input, Price Volatility [Member] | Fuel Oils | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.25		0.22
Measurement Input, Nodal Basis [Member] | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	(1.3)		(1.20)
Measurement Input, Nodal Basis [Member] | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	0.3		0.10
Measurement Input, Nodal Basis [Member] | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	(0.9)		(1.00)
Measurement Input, Nodal Basis [Member] | Natural Gas | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.50)
Measurement Input, Nodal Basis [Member] | Natural Gas | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.30)
Measurement Input, Nodal Basis [Member] | Natural Gas | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.40)
Measurement Input, Nodal Basis [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	(10)		(10)
Measurement Input, Nodal Basis [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	0		0
Measurement Input, Nodal Basis [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	(2)		(2)
Measurement Input, Quoted Price [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	24
Measurement Input, Quoted Price [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	39
Measurement Input, Quoted Price [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	27
Measurement Input, Auction Pricing [Member] | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	(898)		(65)
Measurement Input, Auction Pricing [Member] | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	1,180		1,823
Measurement Input, Auction Pricing [Member] | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	57		251
Measurement Input, Commodity Future Price [Member] | Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3		3
Measurement Input, Commodity Future Price [Member] | Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3		4
Measurement Input, Commodity Future Price [Member] | Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3		3
Measurement Input, Escalation Rate [Member] | Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Measurement Input, Escalation Rate [Member] | Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Measurement Input, Escalation Rate [Member] | Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Measurement Input, Estimated Renewable Energy Credit Costs [Member] | Power | Contract Price Allocation | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	5		5
Measurement Input, Estimated Renewable Energy Credit Costs [Member] | Power | Contract Price Allocation | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	7		7
Measurement Input, Estimated Renewable Energy Credit Costs [Member] | Power | Contract Price Allocation | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	6		6