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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2026	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2020	Jul. 20, 2016	Jan. 17, 2012
Debt Instrument
Issues of power bonds	$ 997	$ 0	$ 998
Face Amount	40										$ 100
Discount on debt issues	(3)	0
Redemptions/Maturities of variable interest entities	39	38	36
Redemptions/Maturities of power bonds	1,427	1,035	1,731
Percent of par value									99.70%
Leaseback obligation settled as a result of acquisition										$ 70
Total Current maturities of power bonds issued at par	1,787	1,030
Interest rate											7.10%
Short-term debt, net of discounts	57	922	$ 1,217	$ 0	$ 0	$ 0	$ 0	$ 0
Current maturities of long-term debt of variable interest entities issued at par	41	39
Current maturities of notes payable	0	23
880591EL2
Debt Instrument
Total Current maturities of power bonds issued at par	$ 1,500	0
Interest rate	3.875%
Debt Instrument, Maturity Date	Feb. 15, 2021
880591DC3
Debt Instrument
Total Current maturities of power bonds issued at par	$ 258	0
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	0
Debt Instrument, Maturity Date	Dec. 15, 2020
880591EF5
Debt Instrument
Total Current maturities of power bonds issued at par	$ 0	1
Interest rate	3.77%
Debt Instrument, Maturity Date	Dec. 15, 2019
880591EF5 (6.15.21)
Debt Instrument
Debt Instrument, Maturity Date	Jun. 15, 2021
880591EF5 (6/15/2018)
Debt Instrument
Total Current maturities of power bonds issued at par	$ 0	27
Debt Instrument, Maturity Date	Jun. 15, 2020
880591EF5 (6/15/2021)
Debt Instrument
Total Current maturities of power bonds issued at par	$ 28	0
PARRS 1998 Series D Bond
Debt Instrument
Redemptions/Maturities of power bonds	23	0
PARRS 1999 Series A Bond
Debt Instrument
Redemptions/Maturities of power bonds	1,000	0
2020 Series A
Debt Instrument
Redemptions/Maturities of power bonds	$ 1,000	$ 0
Total
Debt Instrument
Debt Instrument, Redemption Period, End Date	1,489	1,119
Percent of par value	100.00%
Debt of variable interest entities
Debt Instrument
Redemptions/Maturities of variable interest entities	$ 39	$ 38
Notes Payable
Debt Instrument
Redemptions/Maturities of notes payable	23	46
electronotes
Debt Instrument
Redemptions/Maturities of power bonds	219	5
2013 Series A [Member]
Debt Instrument
Redemptions/Maturities of power bonds		1,000
2009 Series B
Debt Instrument
Redemptions/Maturities of power bonds	28	30
1997 Series E [Member]
Debt Instrument
Redemptions/Maturities of power bonds	17
1995 Series B
Debt Instrument
Redemptions/Maturities of power bonds	140	0
Total
Debt Instrument
Debt Securities Issues	$ 997	$ 0

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