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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, 2020 USD ($) Years | Dec. 31, 2019 USD ($) | Sep. 30, 2020 USD ($)
Derivative |  |  |  |
Amount recognized for unrealized gains (losses) | $ 0 |  |  |
Change in Unrealized gains (losses) on Interest Rate Derivatives | (143,000,000) | $ 171,000,000 |  |
Interest Rate Swap |  |  |  |
Derivative |  |  |  |
Gain (loss) recognized in income on derivatives | (29,000,000) | (21,000,000) | [1] |
Fair value | (533,000,000) |  |  | $ (588,000,000)
Commodity Contract Derivatives |  |  |  |
Derivative |  |  |  |
Gain (loss) recognized in income on derivatives | 0 | $ 1,000,000 |  |
Fair value | $ 22,000,000 |  |  | $ 46,000,000
Natural Gas [Member] |  |  |  |
Derivative |  |  |  |
Number of contracts | 32 |  |  | 42
Derivative, Nonmonetary Notional Amount | 300,000,000 |  |  | 302,000,000
Fair value | $ 22,000,000 |  |  | $ 46,000,000
Number Commodity Contract Term Years | Years | 4 |  |  |
[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 were no related gains (losses) recognized in income for these unrealized gains (losses) for the three months ended December 31, 2020 and 2019.(2) During the fourth quarter of 2020, TVA discontinued derivative accounting for forward coal contracts.

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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=121572278&loc=d3e13433-108611 + Details
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