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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) $ in Millions | 12 Months Ended
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Sep. 30, 2019 USD ($) megawatts Megawatts | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Obligations |  |  |
Megawatts provided under power purchase obligations | 2,230 |  |
Remaining terms of the agreements, high end of range | 13 years |  |
Megawatts provided under transmission obligations | megawatts | 1,450 |  |
Power purchased under agreement | $ | $ 195 | $ 188 | $ 178
Maximum term length for the purchase, storage, and transportation of natural gas | four |  |
Purchase Agreements Required by Federal Law |  |  |
Obligations |  |  |
Megawatts provided under power purchase obligations | 263 |  |

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- Definition Maximum term length for the purchase, storage, and transportation of natural gas + References No definition available. + Details
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Namespace Prefix: | tve_
Data Type: | us-types:durationStringItemType
Balance Type: | na
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- Definition Number of megawatts provided under the power purchase obligations. + References No definition available. + Details
Name: | tve_MegawattsProvidedUnderPowerPurchaseObligations
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
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- Definition Megawatts provided under transmission obligations + References No definition available. + Details
Name: | tve_Megawattsprovidedundertransmissionobligations
Namespace Prefix: | tve_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition The expense for power purchased under contractual power purchase obligations. + References No definition available. + Details
Name: | tve_PowerPurchasedUnderAgreement
Namespace Prefix: | tve_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The high end of the range of remaining terms of the agreements. + References No definition available. + Details
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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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Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |