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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Investment Gains (Losses)
Financing Receivable, after Allowance for Credit Loss	$ 107		$ 107		$ 93
Balance in NDT	$ 2,600		$ 2,600
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
Investment funds	$ 3,723		$ 3,723		$ 3,198
LTDCP [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	0	$ 3	(1)	$ 2
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	(1)	9	(5)	8
ART [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(28)	135	(120)	98
NDT [Member]
Investment Gains (Losses)
Unrealized gains (losses) on investments	(23)	$ 357	253	$ 246
Equity Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	220		220
Real Estate Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	88		88
Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	44		44
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	69		69
Private Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 22		$ 22

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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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