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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)
Investment Gains (Losses)
Debt Securities, Trading, and Equity Securities, FV-NI	$ 2,835		$ 2,835
Financing Receivable, Net	136		136		$ 123
Balance in NDT	2,100		2,100
Balance in ART	$ 698		$ 698
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
SERP
Investment Gains (Losses)
Unrealized gains (losses) on investments	$ 5	$ (2)	$ (1)	$ 0
DCP
Investment Gains (Losses)
Unrealized gains (losses) on investments	1	(1)	(2)	0
NDT
Investment Gains (Losses)
Unrealized gains (losses) on investments	154	(75)	(47)	(29)
ART
Investment Gains (Losses)
Unrealized gains (losses) on investments	52	$ (14)	(52)	$ 6
Equity Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	142		142
Real Estate Funds
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	26		26
Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	11		11
Private Equity Funds [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	59		59
Private real estate funds [Member]
Investment Gains (Losses)
Fair value plan assets gross	11		11
Private Credit [Member]
Investment Gains (Losses)
NDT unfunded commitments related to private equity and real estate	$ 6		$ 6

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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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Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

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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).

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