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Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Sep. 30, 2017
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Regulatory Assets and Liabilities |  |
Current regulatory assets | $ 471 | $ 447
Non-current regulatory assets | 8,240 | 8,698
Regulatory assets | 8,711 | 9,145
Current regulatory liabilities | 159 | 163
Non-current regulatory liabilities | 19 | 25
Regulatory liabilities | 178 | 188
Deferred other post-retirement benefits cost |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory liabilities | 16 | 23
Fuel cost adjustment tax equivalents |  |
Regulatory Assets and Liabilities |  |
Current regulatory liabilities | 147 | 153
Fuel cost adjustment receivable/liability |  |
Regulatory Assets and Liabilities |  |
Current regulatory liabilities | 0 | 2
Unrealized gains/losses on commodity derivatives |  |
Regulatory Assets and Liabilities |  |
Current regulatory liabilities | 12 | 8
Non-current regulatory liabilities | 3 | 2
Deferred nuclear generating units |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 237 | 237
Non-current regulatory assets | 645 | 759
Unrealized losses on interest rate derivatives |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 80 | 93
Non-current regulatory assets | 824 | 982
Unrealized gains/losses on commodity derivatives |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 44 | 68
Non-current regulatory assets | 36 | 9
Fuel cost adjustment receivable/liability |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 39 | 1
Environmental agreements |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 3 | 2
Non-current regulatory assets | 11 | 13
Environmental cleanup costs - Kingston ash spill |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 44 | 44
Non-current regulatory assets | 242 | 263
Gallatin coal combustion residual facilities |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 21 | 0
Non-current regulatory assets | 878 | 899
Other current regulatory assets |  |
Regulatory Assets and Liabilities |  |
Current regulatory assets | 3 | 2
Deferred pension costs and other post-retirement benefits costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 3,881 | 4,009
Nuclear decommissioning costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 821 | 823
Non-nuclear decommissioning costs |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | 701 | 703
Other non-current regulatory assets |  |
Regulatory Assets and Liabilities |  |
Non-current regulatory assets | $ 201 | $ 238

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- Definition Regulatory Assets and Liabilities [Line Items] + References No definition available. + 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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -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 not expected to be recovered through revenue sources within one year or the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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