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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 124		$ 120
Balance in NDT	2,300
Balance in ART	$ 815
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
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ 1	$ (3)
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	1	(6)
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	37	(104)
NDT [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	111	$ (201)
Equity Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	208
Real Estate Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	57
Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	10
Private Equity Funds [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	103
Private real estate funds [Member]
Investment Gains (Losses)
Fair value plan assets gross	41
Private Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 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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Number of readily available quoted exchange prices for the investments

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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, 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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