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Benefit Plans Actuarial Assumptions (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Oct. 01, 2023
Defined Benefit Plan Disclosure
Discount rate		3.70%
Period during which actual company compensation experience study performed	5 years
Defined Benefit Plan, Cost of Living Adjustment Assumption	25.00%
Pension Benefits
Defined Benefit Plan Disclosure
Discount rate	3.85%	3.65%	4.50%
Discount rate	4.35%	3.85%
Rate of compensation increase	4.00%	5.43%
Expected return on plan assets	7.00%	6.75%	7.00%
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Increase (Decrease) for Plan Amendment		$ 6.7500
Defined Benefit Plan, Cost of Living Adjustment Assumption	2.00%	2.00%	2.40%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	5.34%	5.43%	5.55%
COLA percentage increase (decrease)	600.00%
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Discount rate	3.95%	3.70%	4.65%
Discount rate	4.40%	3.95%
Defined Benefit Plan, Cost of Living Adjustment Assumption	2.00%	2.00%	2.40%
Minimum
Defined Benefit Plan Disclosure
Rate of compensation increase	2.50%
Maximum
Defined Benefit Plan Disclosure
Rate of compensation increase	14.00%
Pre-Medicare Eligible [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Initial health care cost trend rate	6.25%	6.50%	7.00%
Ultimate health care cost trend rate	5.00%	5.00%	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2024	2024
Pre-Medicare Eligible [Member] | Scenario, Forecast | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Ultimate health care cost trend rate				5.00%
Post-Medicare Eligible [Member] [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Initial health care cost trend rate	0.00%	0.00%	7.00%
Ultimate health care cost trend rate	4.00%	4.00%	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2021	2021
Health Care [Member] | Pre-Medicare Eligible [Member] | Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Initial health care cost trend rate	6.50%

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- Definition

COLA percentage increase (decrease)

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- Definition

The cost of living adjustment percentage used for defined benefit plans.

+ References

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- Definition

The period in which an actual compensation experience study is performed at TVA to determine assumptions related to compensation increases for defined benefit plans.

+ References

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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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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Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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-Section 50
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- Definition

Amount of increase (decrease) in net periodic benefit cost from effect of change in substantive term of existing defined benefit plan or initiation of new defined benefit plan.

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Ultimate trend rate for health care cost for defined benefit postretirement plan.

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Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

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