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Benefit Plans Components of Benefit Plans (Details)	12 Months Ended				240 Months Ended
	Sep. 30, 2017	Sep. 30, 2016 USD ($) plans Years	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2036 USD ($)	Oct. 01, 2016 Years
Defined Benefit Plan Disclosure
Number of unfunded post-retirement health care plans | plans		2
Fixed and variable fund annual maximum contribution		$ 10,000
Age Threshold for Changes to COLA Changes for Cash Balance Plan | Years						50
Defined contribution plan contribution amount		$ 38,000,000	$ 36,000,000	$ 35,000,000
Market-related value phase-in period number of years | Years		3
Minimum
Defined Benefit Plan Disclosure
Threshold for Deferral of Actuarial Gain/Loss Under Corridor Approach		10.00%
Scenario, Forecast
Defined Benefit Plan Disclosure
Minimum Funding for TVA Pension Plan Under 2016 Amendments					$ 300,000,000
Funded Status Threshold to Reduce Minimum Contribution Under 2016 Changes to TVA Pension Plan					100.00%
Scenario, Forecast | Maximum
Defined Benefit Plan Disclosure
Years of Service Threshold for Changes to Cash Balance Plan	10 years

X

- Definition

Age Threshold for Changes to COLA Related to 2016 Amendments to TVA Pension Plan

+ References

No definition available.

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- Definition

The maximum annual contribution that employees can make to the fixed and variable fund benefit. The fixed fund benefit is a benefit that earns a specified rate of interest on contributions made by employees. The variable fund is a benefit that invests employee contributions in the Spartan U.S. Equity Index Fund-Fidelity Advantage Class (Spartan).

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- Definition

Funded Status Threshold to Reduce Minimum Contribution Under 2016 Changes to TVA Pension Plan

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- Definition

The number of years for the phase-in period in which the impact of asset performance on pension expense is phased-in through a market-related value of asset calculation.

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- Definition

Minimum Funding for TVA Pension Plan Under 2016 Changes

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- Definition

The number of unfunded post-retirement health care plans that provide for non-vested contributions toward the cost of certain retirees' medical coverage.

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No definition available.

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- Definition

Threshold for Deferral of Actuarial Gain/Loss Under Corridor Approach

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- Definition

Years of Service Threshold Related to 2016 Amendments to TVA Pension Plan

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No definition available.

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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.

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No definition available.

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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