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Debt and Other Obligations Debt and Other Obligations - Debt Securities Activity (Details) $ in Millions		12 Months Ended
	Sep. 20, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Feb. 07, 2017 USD ($)	Jul. 20, 2016 USD ($)	Sep. 30, 2003 Units
Debt Instrument
Number of Businesses Acquired	2
Leaseback transaction, number of units | Units							24
Issues of power bonds		$ 998	$ 999	$ 0
Discount on debt issues			(1)		$ (2)
Acquisition of notes payable			74	78
Redemptions/Maturities of variable interest entities		36	35	33
Redemptions/Maturities of power bonds		$ 1,731	$ 1,558	$ 76
Leaseback obligation settled as a result of acquisition	$ 70					$ 70
Total
Debt Instrument
Debt Instrument, Redemption Period, End Date		1820	1620
Percent of par value		100.00%
Debt of variable interest entities
Debt Instrument
Redemptions/Maturities of variable interest entities		$ 36	$ 35
Notes Payable
Debt Instrument
Redemptions/Maturities of notes payable		53	27
electronotes
Debt Instrument
Redemptions/Maturities of power bonds		52	5
2009 Series B
Debt Instrument
Redemptions/Maturities of power bonds		29	28
2001 Series D
Debt Instrument
Redemptions/Maturities of power bonds		0	525
2007 Series A
Debt Instrument
Redemptions/Maturities of power bonds		0	1,000
1997 Series E [Member]
Debt Instrument
Redemptions/Maturities of power bonds		650	0
2008 Series B [Member]
Debt Instrument
Redemptions/Maturities of power bonds		1,000	0
Notes payable
Debt Instrument
Acquisition of notes payable		0	74
Total
Debt Instrument
Debt Securities Issues		998	999
2017 Series A
Debt Instrument
Issues of power bonds		$ 0	$ 1,000
Percent of par value		99.80%

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