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Summary of Significant Accounting Policies General and Basis of Presentation (Details) People in Millions, $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 USD ($) People | Mar. 31, 2017 USD ($) | Mar. 31, 2018 USD ($) People | Mar. 31, 2017 USD ($) | Sep. 30, 2017 USD ($)
Accounting Policies [Abstract] |  |  |  |  |
Loans receivable, carrying value | $ 129 |  | $ 129 |  | $ 107
Population of TVA's service area | People | 9 |  | 9 |  |
Allowance for uncollectible accounts, accounts receivable | $ 1 |  | $ 1 |  | 1
Allowance for loans receivable | 1 |  | 1 |  |
Revenue capitalized during pre-commercial operations | 10 | $ 6 | 11 | $ 20 |
Fuel cost capitalized during pre-commercial operations | 11 | 8 | 14 |  |
Depreciation expense | 331 | $ 336 | 649 | $ 672 |
Financing Receivable, Recorded Investment, Current | $ 3 |  | $ 3 |  | $ 3

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- Definition Fuel costs incurred during pre-commercial generation capitalized to project costs. + References No definition available. + Details
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- Definition The population of the company's service area. + References No definition available. + Details
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- Definition Estimates of revenue realized from pre-commercial generation based on the guidance provided by FERC regulations. The calculated revenue is capitalized to offset project costs and is included in revenue as a contra-revenue amount + References No definition available. + Details
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- Definition A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible. + 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 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 + Details
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