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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Debt Instrument
Redemptions/maturities of power bonds		$ 1,002	[1]	$ 698
Total redemptions/maturities of debt	[1]	(1,003)
Repayments of Notes Payable		1		$ 2
2013 Series A [Member]
Debt Instrument
Redemptions/maturities of power bonds		1,000
2009 Series B [Member]
Debt Instrument
Redemptions/maturities of power bonds		1
Notes Payable [Member]
Debt Instrument
Repayments of Notes Payable		1
electronotes [Member]
Debt Instrument
Redemptions/maturities of power bonds		$ 1
Total
Debt Instrument
Percent of par value		100.00%
[1] (1) All redemptions were at 100 percent of par.

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The percent of par value that debt was issued/redeemed for.

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- Definition

Amount of cash inflow (outflow) for debt classified as other.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
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The cash outflow for a borrowing supported by a written promise to pay an obligation.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
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The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
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