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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details)	12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Derivative
Unrealized gains/losses on derivatives	$ 0
Change in Unrealized gains (losses) on Interest Rate Derivatives	565,000,000	$ (310,000,000)
Interest rate swaps	1,764,000,000	1,199,000,000
Interest Rate Swap
Derivative
Amount of gain (loss) recognized in income on derivatives	(79,000,000)	(89,000,000)
Fair value	(498,000,000)	(317,000,000)
Commodity Contract Derivatives
Derivative
Fair value	(41,000,000)	60,000,000
Commodity derivatives under FTP
Derivative
Amount of gain (loss) recognized in income on derivatives	$ 0	$ (8,000,000)
Coal Contract Derivatives
Derivative
Number of contracts	8	13
Notional amount	9,000,000	20,000,000
Fair value	$ (4,000,000)	$ 58,000,000
Natural Gas Contract Derivatives
Derivative
Number of contracts	65	61
Notional amount	330,000,000	359,000,000
Fair value	$ (37,000,000)	$ 2,000,000
Accounts payable and accrued liabilities
Derivative
Interest rate swaps	88,000,000	77,000,000
Accounts payable and accrued liabilities | Interest Rate Swap
Derivative
Fair value	(24,000,000)	(20,000,000)
Accounts payable and accrued liabilities | Commodity Contract Derivatives
Derivative
Fair value	$ (37,000,000)	$ (4,000,000)

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of interest rate derivatives held at each balance sheet date.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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