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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, 2018 USD ($) | Sep. 30, 2017 USD ($)
Derivative |  |
Unrealized gains/losses on derivatives | $ 0 |
Change in Unrealized gains (losses) on Interest Rate Derivatives | 310,000,000 | $ (472,000,000)
FTP transaction limit | $ 130,000,000 |
Maximum hedge volume | 75.00% |
Market value limitation of outstanding construction materials hedging transactions | $ 100,000,000 |
Portfolio value at risk limit for foreign currency transactions | 5,000,000 |
Interest Rate Swap |  |
Derivative |  |
Amount of gain (loss) recognized in income on derivatives | (89,000,000) | (101,000,000)
Fair value | (317,000,000) | (410,000,000)
Commodity Contract Derivatives |  |
Derivative |  |
Fair value | 60,000,000 | (60,000,000)
Commodity derivatives under FTP |  |
Derivative |  |
Amount of gain (loss) recognized in income on derivatives | (8,000,000) | (36,000,000)
Fair value | $ 0 | $ (5,000,000)
Coal Contract Derivatives |  |
Derivative |  |
Number of contracts | 13 | 20
Notional amount | 20,000,000 | 17,000,000
Fair value | $ 58,000,000 | $ (67,000,000)
Natural Gas Contract Derivatives |  |
Derivative |  |
Number of contracts | 61 | 53
Notional amount | 359,000,000 | 271,000,000
Fair value | $ 2,000,000 | $ 7,000,000
Natural Gas Swap |  |
Derivative |  |
Notional amount | 0 | 2,800,000
Fair value | $ 0 | $ (5,000,000)

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