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Impact of New Accounting Standards and Interpretations Impact of New Accounting Standards and Interpretations (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification from Operating and maintenance expense to Other income | $ 758 | $ 179 | $ 185
Reclassification of net periodic benefit costs |  | 256 | 758
Reclassification of cash receipts (payments) |  | $ 29 | $ 9

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- Definition Reclassification from Operating and maintenance expense to Other income + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Reclassification of cash receipts (payments) + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Reclassification of net periodic benefit costs + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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Period Type: | duration