Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Sep. 30, 2021 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2020 USD ($) $ / MWh $ / MMBTU
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 21	$ 5
Derivative liabilities \| $	$ (188)	$ (201)
Commodity Forward Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	28	23
Commodity Forward Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	77	37
Commodity Forward Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	40	29
Nodal Basis \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(4)	(6)
Nodal Basis \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0	0
Nodal Basis \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(2)	(2)
Commodity Future Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(4)	2
Commodity Future Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(2)	6
Commodity Future Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	(3)	3

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Value of input used to measure net derivative asset (liability).

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