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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Investment fund securities classified as trading and measured at fair value | $ 2,900 |
Balance in the NDT | 2,100 |
Balance in the ART | $ 714 |
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 | $ 8,423 | $ 8,536
Number of readily available quoted exchange prices for the investments | 0 |
SERP |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | $ 1 | 4
DCP |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 1 | 2
NDT |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 18 | 92
ART |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Unrealized gains (losses) on investments | 15 | $ 43
Equity Funds [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 119 |
Real Estate Funds [Member] |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 28 |
Private equity |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Commitments, Fair Value Disclosure | 38 |
Private real estate funds |  |
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items] |  |
Fair value of gross plan assets | 12 |
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 Minimum investment period of private partnership investments. + 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 nuclear facilities. + References No definition available. + Details
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- Definition Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(25)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + 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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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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