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Risk Management Activities and Derivative Transactions Derivative Instruments That Do Not Receive Hedge Accounting Treatment (Details) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Sep. 30, 2017 USD ($)
Derivative |  |  |  |  |  |  |  |
Amount recognized for unrealized gains (losses) |  |  |  | $ 0 |  |  |  |
Change in Unrealized gains (losses) on Interest Rate Derivatives |  | $ (143,000,000) | $ 18,000,000 | (171,000,000) |  | $ 459,000,000 |  |
Interest Rate Swap |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Gain (loss) recognized in income on derivatives | [1] | (23,000,000) | (26,000,000) | (47,000,000) |  | (52,000,000) |  |
Fair value |  | (359,000,000) |  | (359,000,000) |  |  |  | $ (410,000,000)
Commodity Contract Derivatives |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Gain (loss) recognized in income on derivatives |  | 3,000,000 | 2,000,000 | 0 | [1] | 0 | [1] |
Fair value |  | (65,000,000) |  | (65,000,000) |  |  |  | (60,000,000)
Commodity derivatives under the financial trading program |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Gain (loss) recognized in income on derivatives | [1] | 0 | $ (6,000,000) | (8,000,000) |  | $ (21,000,000) |  |
Fair value | [2] | $ 0 |  | $ 0 |  |  |  | $ (5,000,000)
Coal Contract Derivatives |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Number of contracts |  | 12 |  | 12 |  |  |  | 20
Notional amount |  | 20,000,000 |  | 20,000,000 |  |  |  | 17,000,000
Fair value |  | $ (52,000,000) |  | $ (52,000,000) |  |  |  | $ (67,000,000)
Natural gas contract derivatives |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Number of contracts |  | 35 |  | 35 |  |  |  | 53
Notional amount |  | 299,000,000 |  | 299,000,000 |  |  |  | 271,000,000
Fair value |  | $ (13,000,000) |  | $ (13,000,000) |  |  |  | $ 7,000,000
Natural gas contract derivatives |  |  |  |  |  |  |  |
Derivative |  |  |  |  |  |  |  |
Fair value |  | $ 0 |  | $ 0 |  |  |  | $ (5,000,000)
[1] | All of TVA's derivative instruments that do not receive hedge accounting treatment have unrealized gains (losses) that would otherwise be recognized in incomebut 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 and six months ended March 31, 2018 and 2017.
[2] | Fair values of certain derivatives under the FTP that were in net liability positions totaling $4 million at September 30, 2017, were recorded in TVA's margin cash accounts in Other current assets. These derivatives were transacted with futures commission merchants, and cash deposits were posted to the margin cash accounts held with each futures commission merchant to offset the net liability positions in full. At March 31, 2018, TVA had no derivatives under the FTP in net liability positions.

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