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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details) | 3 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2018 USD ($)
Derivative |  |  |  |  |
Amount recognized for unrealized gains (losses) |  | $ 0 |  |  |
Change in Unrealized gains (losses) on Interest Rate Derivatives |  | (110,000,000) | $ 28,000,000 |  |
Interest Rate Swap |  |  |  |  |
Derivative |  |  |  |  |
Gain (loss) recognized in income on derivatives |  | (20,000,000) | (24,000,000) | [1] |
Fair value |  | (343,000,000) |  |  | $ (317,000,000)
Commodity Contract Derivatives |  |  |  |  |
Derivative |  |  |  |  |
Gain (loss) recognized in income on derivatives |  | 0 | (3,000,000) |  |
Fair value |  | 67,000,000 |  |  | $ 60,000,000
Commodity derivatives under the financial trading program |  |  |  |  |
Derivative |  |  |  |  |
Gain (loss) recognized in income on derivatives | [1] | $ 0 | $ 8,000,000 |  |
Coal Contract Derivatives |  |  |  |  |
Derivative |  |  |  |  |
Number of contracts |  | 16 |  |  | 13
Notional amount |  | 17,000,000 |  |  | 20,000,000
Fair value |  | $ 63,000,000 |  |  | $ 58,000,000
Natural gas contract derivatives |  |  |  |  |
Derivative |  |  |  |  |
Number of contracts |  | 53 |  |  | 61
Notional amount |  | 347,000,000 |  |  | 359,000,000
Fair value |  | $ 4,000,000 |  |  | $ 2,000,000
[1] | All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in income but instead are deferred as regulatory assets and liabilities. As such, there was no related gain (loss) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2018 and 2017.

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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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. + References No definition available. + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611 + Details
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