Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) - Power $ in Millions	Sep. 30, 2019 USD ($) $ / MWh $ / MMBTU	Sep. 30, 2018 USD ($) $ / MWh $ / MMBTU
Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets \| $	$ 11	$ 3
Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities \| $	$ (207)	$ (186)
Measurement Input, Quoted Price [Member] \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	22
Measurement Input, Quoted Price [Member] \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	37
Commodity Forward Price \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input		23
Commodity Forward Price \| Discounted Cash Flow \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input		39
Commodity Forward Price \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	25	28
Nodal Basis \| Discounted Cash Flow \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	(7)	(9)
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	(3)	(2)
Commodity Future Price \| Fundamental Energy Production Model \| Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	3
Commodity Future Price \| Fundamental Energy Production Model \| Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	4
Commodity Future Price \| Fundamental Energy Production Model \| Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input \| $ / MMBTU	3	3

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