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Fair Value Measurements - Investments (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Long-term Investments | $ 3,198 | $ 2,968
Balance in the NDT | 2,200 |
Balance in the ART | $ 866 |
Period of time where the investor contributes capital to an investment in a private partnership - minimum | three |
Period of time where the investor contributes capital to an investment in a private partnership - maximum | four |
Minimum investment period | 10 years |
Fair value of gross plan assets | $ 8,368 | 8,472
Number of readily available quoted exchange prices for the investments | 0 |
Increase in NDT | $ 123 |
LTDCP |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 2 | (2)
SERP |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 3 | 0
ART |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 32 | (70)
NDT |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 37 | $ (112)
Equity Funds [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 218 |
Real Estate Funds [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 67 |
Credit [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 33 |
Private equity |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 133 |
Private real estate funds |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Fair value of gross plan assets | 54 |
Private Credit [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 16 |
Interest Payments relating to energy prepayment obligations |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
2019 | $ 4 |

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. This amount is not reduced by non-benefit liabilities. + References No definition available. + Details
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- Definition The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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