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Risk Management Activities and Derivative Transactions Currency Swaps Outstanding (Details) $ in Millions	9 Months Ended
	Jun. 30, 2018 USD ($) Bond_issues
Derivative
Number of British pound sterling denominated bond transactions	3
Number of currency swaps outstanding	3
Associated TVA bond issues currency exposure | $	$ 600
Minimum
Derivative
Expiration date range of swaps	2021
Maximum
Derivative
Expiration date range of swaps	2043

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

No definition available.

+ Details

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- Definition

Expiration Date of Swap

+ References

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+ Details

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- Definition

Number of British pound streling denominated bond transactions

+ References

No definition available.

+ Details

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- Definition

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.

+ References

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- Definition

Number of foreign currency exchange rate derivatives held by the entity at the reporting date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

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