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Benefit Plans Actuarial Assumptions (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Defined Benefit Plan Disclosure |  |  |
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 | 4.35% | 3.85% | 3.65%
Discount rate | 3.20% | 4.35% | 3.85%
Rate of compensation increase | 4.00% | 3.60% |
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% | 2.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | 3.50% | 5.34% | 5.43%
COLA percentage increase (decrease) | 600.00% |  |
Other Post-retirement Benefits |  |  |
Defined Benefit Plan Disclosure |  |  |
Discount rate | 4.40% | 3.95% | 3.70%
Discount rate | 3.30% | 4.40% |
Defined Benefit Plan, Cost of Living Adjustment Assumption | 2.00% | 2.00% | 2.00%
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% | 6.50%
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 | 2027 | 2024 |
Initial health care cost trend rate | $ 0.0675 | $ 0.0625 |
Post-Medicare Eligible [Member] [Member] | Other Post-retirement Benefits |  |  |
Defined Benefit Plan Disclosure |  |  |
Initial health care cost trend rate | 0.00% | 0.00% | 0.00%
Ultimate health care cost trend rate | 4.00% | 4.00% | 4.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate | 2023 | 2021 |

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- Definition COLA percentage increase (decrease) + References No definition available. + Details
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- Definition The cost of living adjustment percentage used for defined benefit plans. + References No definition available. + Details
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- Definition Initial health care cost trend rate + References No definition available. + Details
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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 No definition available. + Details
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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 Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (r) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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