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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions		6 Months Ended
		Mar. 31, 2025		Mar. 31, 2024	Oct. 02, 2024	Sep. 30, 2024
Debt Instrument
Total long-term debt, net		$ 19,385				$ 18,764
Repayments of Unsecured Debt		1	[1]	$ 1
Amount of letters of credit outstanding		$ 6
Percent of par value	[1]	100.00%
Long-term power bonds, net		$ 17,729				$ 17,867
Repayments of Other Long-term Debt		$ 18		$ 17
Debt Instrument, Redemption, Description		19
Proceeds from Issuance of Debt		$ 2,031
Debt Instrument, Unamortized Discount (Premium), Net		$ 19
880591FC1
Debt Instrument
Percent of par value		98.517%
Long-term power bonds, net		$ 1,250
Variable Interest Entity, Primary Beneficiary [Member]
Debt Instrument
Debt and Lease Obligation					$ 800
2009 series B
Debt Instrument
Repayments of Unsecured Debt		$ 1
[1] (2) All redemptions were at 100 percent of par.

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