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Regulatory Assets and Liabilities Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017 | Aug. 01, 2017
Regulatory Assets and Liabilities |  |  |  |  |
Non-current regulatory liabilities |  | $ 0 | $ 104 |  |
Accelerated Amortization of Deferred Nuclear Generating Units and Training Costs |  | 857 |  |  |
Accelerated recovery for Kingston ash spill |  | 266 |  |  |
Current regulatory assets |  | 156 | 414 |  |
Regulatory assets |  | 8,763 | 6,612 |  |
Regulatory Assets |  | 8,919 | 7,026 |  |
Current regulatory liabilities |  | 150 | 187 |  |
Regulatory asset amount expensed |  | 261 | 2 | $ 143 |
Gallatin CCR | [1] | $ 672 |  |  |
Gallatin CCR regulatory asset |  |  |  |  | $ 900
Period collected in rates |  | 15 years |  |  |
Regulatory Liabilities |  | $ 150 | 291 |  |
Environmental Agreements |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Initial liability recorded |  | 360 |  |  |
Investment in environmental agreement projects |  | 290 |  |  |
Amount to be provided to fund environmental projects |  | 60 |  |  |
Amount to be paid in civil penalties |  | $ 10 |  |  |
Environmental cleanup costs - Kingston ash spill |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory asset amortization period |  | 12 months |  |  |
Energy Efficiency Projects | Environmental Agreements |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory asset amount expensed |  | $ 279 |  |  |
Paid to States | Environmental Agreements |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory asset amount expensed |  | 60 |  |  |
Civil Penalties | Environmental Agreements |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory asset amount expensed |  | 10 |  |  |
Deferred Project Costs [Member] |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Gallatin coal combustion residual facilities estimated cost to cap and close |  | 89 | 73 |  |
Regulatory Asset |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory assets |  | 130 | 118 |  |
Removal Costs [Member] |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory assets |  | 1,241 | 692 |  |
Environmental Agreements |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory assets |  | 0 | 3 |  |
Regulatory assets |  | 12 | 11 |  |
Unrealized losses on interest rate derivatives |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory assets |  | 39 | 4 |  |
Regulatory assets |  | 15 | 8 |  |
Non-nuclear decommissioning costs |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory assets |  | 1,741 | 1,019 |  |
Pension Costs [Member] |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory assets |  | 4,756 | 3,119 |  |
Unrealized gains/losses on commodity derivatives |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory assets |  | 0 | 38 |  |
Regulatory assets |  | 0 | 861 |  |
Deferred Fuel Costs [Member] |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory assets |  | 28 | 30 |  |
Environmental cleanup costs - Kingston ash spill |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory assets |  | 0 | 266 |  |
Nuclear decommissioning costs |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Regulatory assets |  | 868 | 784 |  |
Other current regulatory assets |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Gallatin CCR regulatory asset |  |  | 38 |  |
Regulatory Asset |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Gallatin CCR regulatory asset |  |  | 861 |  |
Unrealized gains/losses on commodity derivatives |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Non-current regulatory liabilities |  | 0 | 31 |  |
Current regulatory liabilities |  | 12 | 41 |  |
Deferred other post-retirement benefits cost |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Non-current regulatory liabilities |  | 0 | 73 |  |
Fuel cost adjustment tax equivalents |  |  |  |  |
Regulatory Assets and Liabilities |  |  |  |  |
Current regulatory liabilities |  | $ 138 | $ 146 |  |
[1] | (1) The current portions of the ARO liability in the amounts of $163 million and $115 million at September 30, 2019 and 2018, respectively, are included in Accounts payable and accrued liabilities.

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- Definition The amount of Accelerated Amortization of Deferred Nuclear Generating Units and Training Costs that was taken during the fiscal year. + References No definition available. + Details
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- Definition Amount of environmental agreements to be paid in civil penalties. + References No definition available. + Details
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- Definition Amount to be provided to fund environmental projects in Alabama, Kentucky, North Carolina, and Tennessee + References No definition available. + Details
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- Definition Gallatin CCR + References No definition available. + Details
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- Definition Gallatin CCR regulatory asset + References No definition available. + Details
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- Definition Gallatin coal combustion residual facilities estimated cost to cap and close + References No definition available. + Details
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- Definition The total initial liability recorded for environmental agreements. This was deferred as a regulatory asset, after board approval, to collect this amount in rates. + References No definition available. + Details
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- Definition The amortization period for recovery of the individual regulatory current asset in a schedule of regulatory assets. + References No definition available. + Details
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- Definition The expense charged against earnings during the period to allocate the capitalized costs of regulatory assets over the periods expected to benefit from such costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.3) -URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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