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Debt and Other Obligations Debt and Other Obligations - Debt Outstanding (Details) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2024	Sep. 30, 2023
Short-term debt
Short-term debt, net of discounts		$ 1,167	$ 432
Current maturities of long-term debt of variable interest entities issued at par		37	35
Total Current maturities of power bonds issued at par		1,022	1,022
Total current debt outstanding, net		2,226	1,489
Long-term debt
Long-term power bonds, net		17,867	17,844
Long-term power bonds	[1]	17,995	17,970
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		(128)	(126)
Long-term debt of variable interest entities, net		897	933
Total long-term debt, net		$ 18,764	18,777
880591DX7
Short-term debt
Coupon rate		4.65%
Debt Instrument, Maturity Date		Jun. 15, 2035
Long-term debt
Long-term power bonds, net		$ 436	436
880591EF5
Short-term debt
Coupon rate		3.77%
Debt Instrument, Maturity Date		Jun. 15, 2034
Long-term debt
Long-term power bonds, net		$ 116	139
880591CJ9
Short-term debt
Coupon rate		6.75%
Debt Instrument, Maturity Date		Nov. 01, 2025
Long-term debt
Long-term power bonds, net		$ 1,350	1,350
880591EU2 [Member]
Short-term debt
Coupon rate		2.875%
Debt Instrument, Maturity Date		Feb. 01, 2027
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591300
Short-term debt
Coupon rate		2.134%
Debt Instrument, Maturity Date		Jun. 01, 2028
Long-term debt
Long-term power bonds, net		$ 256	256
880591409
Short-term debt
Coupon rate		2.216%
Debt Instrument, Maturity Date		May 01, 2029
Long-term debt
Long-term power bonds, net		$ 208	208
880591DM1
Short-term debt
Coupon rate		7.125%
Debt Instrument, Maturity Date		May 01, 2030
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591DV1
Short-term debt
Coupon rate		4.70%
Debt Instrument, Maturity Date		Jul. 15, 2033
Long-term debt
Long-term power bonds, net		$ 472	472
880591DP4
Short-term debt
Coupon rate		6.587%
Debt Instrument, Maturity Date		Jun. 07, 2032
Long-term debt
Long-term power bonds, net		$ 335	305
880591CK6
Short-term debt
Coupon rate		5.98%
Debt Instrument, Maturity Date		Apr. 01, 2036
Long-term debt
Long-term power bonds, net		$ 121	121
880591CS9
Short-term debt
Coupon rate		5.88%
Debt Instrument, Maturity Date		Apr. 01, 2036
Long-term debt
Long-term power bonds, net		$ 1,500	1,500
880591CP5
Short-term debt
Coupon rate		6.15%
Debt Instrument, Maturity Date		Jan. 15, 2038
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591ED0
Short-term debt
Coupon rate		5.50%
Debt Instrument, Maturity Date		Jun. 15, 2038
Long-term debt
Long-term power bonds, net		$ 500	500
880591EH1
Short-term debt
Coupon rate		5.25%
Debt Instrument, Maturity Date		Sep. 15, 2039
Long-term debt
Long-term power bonds, net		$ 2,000	2,000
880591EP3
Short-term debt
Coupon rate		3.50%
Debt Instrument, Maturity Date		Dec. 15, 2042
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591DU3
Short-term debt
Coupon rate		4.962%
Debt Instrument, Maturity Date		Jun. 07, 2043
Long-term debt
Long-term power bonds, net		$ 201	183
880591EB4
Short-term debt
Coupon rate		4.875%
Debt Instrument, Maturity Date		Jan. 15, 2048
Long-term debt
Long-term power bonds, net		$ 500	500
880591DZ2
Short-term debt
Coupon rate		5.375%
Debt Instrument, Maturity Date		Apr. 01, 2056
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591EJ7
Short-term debt
Coupon rate		4.625%
Debt Instrument, Maturity Date		Sep. 15, 2060
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591ES7
Short-term debt
Coupon rate		4.25%
Debt Instrument, Maturity Date		Sep. 15, 2065
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591EW8 [Member]
Short-term debt
Coupon rate		0.75%
Debt Instrument, Maturity Date		May 15, 2025
Long-term debt
Long-term power bonds, net		$ 0	1,000
880591EX6
Short-term debt
Coupon rate		1.50%
Debt Instrument, Maturity Date		Sep. 15, 2031
Long-term debt
Long-term power bonds, net		$ 500	500
880591EY4
Short-term debt
Coupon rate		4.25%
Debt Instrument, Maturity Date		Sep. 15, 2052
Long-term debt
Long-term power bonds, net		$ 500	500
880591EZ1
Short-term debt
Coupon rate		3.875%
Debt Instrument, Maturity Date		Mar. 15, 2028
Long-term debt
Long-term power bonds, net		$ 1,000	1,000
880591FB3
Short-term debt
Coupon rate		4.375%
Debt Instrument, Maturity Date		Aug. 01, 2034
Long-term debt
Long-term power bonds, net		$ 1,000	$ 0
[1] nclude total net exchange gain from currency transactions of $62 million and $109 million at September 30, 2024 and 2023

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