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Debt and Other Obligations Debt Securities Activity (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Debt Instrument
Redemptions/maturities of power bonds		$ 698	[1]	$ 527
Total redemptions/maturities of debt	[1]	700
Repayments of Notes Payable		2		$ 0
electronotes
Debt Instrument
Redemptions/maturities of power bonds		$ 47
Interest rate		4.10%
2009 Series B
Debt Instrument
Redemptions/maturities of power bonds		$ 1
Interest rate		3.77%
Total
Debt Instrument
Percent of par value		100.00%
Notes payable
Debt Instrument
Interest rate		1.64%
Repayments of Notes Payable		$ 2
1997 Series E
Debt Instrument
Redemptions/maturities of power bonds		$ 650
Interest rate		6.25%
[1] All redemptions were at 100 percent of par.

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- Definition

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

+ References

No definition available.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The cash outflow for a borrowing supported by a written promise to pay an obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

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- Definition

The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

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