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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2013	Jan. 17, 2012
Debt Instrument
Issues of power bonds	$ 500	$ 997	$ 0
Face Amount	40			$ 360
Discount on debt issues	0	(3)
Redemptions/Maturities of variable interest entities	41	39	38
Redemptions/Maturities of power bonds	$ 1,860	$ 1,427	$ 1,035
Percent of par value	9998.20%	9970.60%
Total Current maturities of power bonds issued at par	$ 1,028	$ 1,787
Interest rate					7.10%
Short-term debt, net of discounts	780	57
Current maturities of long-term debt of variable interest entities issued at par	43	41
Current maturities of notes payable	0	0
880591EL2
Debt Instrument
Total Current maturities of power bonds issued at par	$ 0	1,500
Interest rate	3.875%
Debt Instrument, Maturity Date	Feb. 15, 2021
880591DC3
Debt Instrument
Total Current maturities of power bonds issued at par	$ 0	258
Interest rate	5.805%
Debt Instrument, Maturity Date	Jun. 07, 2021
880591EF5 (12.15.20)
Debt Instrument
Total Current maturities of power bonds issued at par	$ 1	1
Interest rate	3.77%
Debt Instrument, Maturity Date	Dec. 15, 2021
880591EF5 (6.15.21)
Debt Instrument
Total Current maturities of power bonds issued at par	$ 27	28
Interest rate	3.77%
Debt Instrument, Maturity Date	Jun. 15, 2022
880591EN8
Debt Instrument
Total Current maturities of power bonds issued at par	$ 1,000	0
Interest rate	1.875%
Debt Instrument, Maturity Date	Aug. 15, 2022
PARRS 1998 Series D Bond
Debt Instrument
Redemptions/Maturities of power bonds	$ 0	17
PARRS 1999 Series A Bond
Debt Instrument
Redemptions/Maturities of power bonds	$ 0	$ 23
Total
Debt Instrument
Debt Instrument, Redemption Period, End Date	1,901	1,489
Percent of par value	10000.00%
Debt of variable interest entities
Debt Instrument
Redemptions/Maturities of variable interest entities	$ 41	$ 39
Notes Payable
Debt Instrument
Redemptions/Maturities of notes payable	0	23
electronotes
Debt Instrument
Redemptions/Maturities of power bonds	0	219
2009 Series B
Debt Instrument
Redemptions/Maturities of power bonds	29	28
1995 Series B
Debt Instrument
Redemptions/Maturities of power bonds	0	140
2011 Series A
Debt Instrument
Redemptions/Maturities of power bonds	1,500	0
2018 Series A
Debt Instrument
Redemptions/Maturities of power bonds	0	1,000
1998 Series H
Debt Instrument
Redemptions/Maturities of power bonds	331	0
Total
Debt Instrument
Debt Securities Issues	500	997
2020 Series A
Debt Instrument
Debt Securities Issues	0	1,000
2021 Series A
Debt Instrument
Debt Securities Issues	$ 500	$ 0

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