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Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022
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Fair Value Measurements |  |
Commodity contract derivatives, assets | $ 33 | $ 152
Commodity contract derivatives, liabilities | 2 | 7
Fair Value, Inputs, Level 3 |  |
Fair Value Measurements |  |
Commodity contract derivatives, assets | 0 | 0
Commodity contract derivatives, liabilities | $ 0 | $ 0

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- Definition The fair value as of the balance sheet date of firm obligations in physical commodity contracts (for example, agricultural products, metals and energy). + References No definition available. + Details
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Namespace Prefix: | tve_
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Balance Type: | credit
Period Type: | instant

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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_
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Balance Type: | na
Period Type: | duration

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- Definition The fair value as of the balance sheet date of firm holdings in physical commodity contracts (for example, agricultural products, metals and energy). Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings. + References No definition available. + Details
Name: | us-gaap_FinancialInstrumentsOwnedPhysicalCommoditiesAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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