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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
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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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