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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 128		$ 128		$ 120
Balance in NDT	1,900		1,900
Balance in ART	$ 677		$ 677
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
Number of readily available quoted exchange prices for the investments	0		0
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ 3	$ (1)	$ 2	$ 2
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	9	(5)	8	1
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	135	(52)	98	52
NDT [Member]
Investment Gains (Losses)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease)			258
Unrealized gains (losses) on investments	357	$ (154)	246	$ 47
Equity Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	209		209
Real Estate Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	63		63
Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	10		10
Private Equity Funds [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	129		129
Private real estate funds [Member]
Investment Gains (Losses)
Fair value plan assets gross	52		52
Private Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 5		$ 5

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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.

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The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning nuclear facilities.

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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.

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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.

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Amount of increase (decrease) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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