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Benefit Plans Actuarial Assumptions (Details) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2021 | Sep. 30, 2017 | Sep. 30, 2016
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Rate of compensation increase |  |  | 5.70% |  |  |  |  |
Expected return on plan assets |  |  | 7.25% |  |  |  |  |
Period during which actual company compensation experience study performed |  | 5 years |  |  |  |  |  |
Rate of compensation increase |  | 5.70% | 5.72% | 4.44% |  |  |  |
Minimum percentage increase in twelve-month average CPI-U necessary to receive COLA |  | 1.00% |  |  |  |  |  |
Cost of living adjustment assumption |  |  |  |  | 2.50% |  |  |
Pension Benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Discount rate |  | 4.50% | 4.45% | 5.00% |  |  |  |
Rate of compensation increase |  | 5.70% | 5.70% |  |  |  |  |
Discount rate |  | 4.45% | 5.00% |  |  |  |  |
Expected return on plan assets |  | 7.00% | 7.25% |  |  |  |  |
Rate of compensation increase |  | 5.70% | 5.72% |  |  |  |  |
Other Post-retirement Benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Discount rate |  | 4.65% | 4.50% | 5.05% |  |  |  |
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year |  | 7.00% | 7.50% | 8.00% | 8.50% |  |  |
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate |  | 5.00% | 5.00% | 5.00% |  |  |  |
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate |  | 2019 | 2019 |  |  |  |  |
Discount rate |  | 4.50% | 5.05% |  |  |  |  |
Minimum |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Rate of compensation increase |  | 3.50% |  |  |  |  |  |
Cost of living adjustment assumption |  | 1.00% |  |  |  |  |  |
Target Fed Funds Rate |  | 0.00% |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Rate of compensation increase |  | 13.00% |  |  |  |  |  |
Cost of living adjustment assumption |  | 5.00% |  |  |  |  |  |
Target Fed Funds Rate |  | 0.25% |  |  |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Cost of living adjustment assumption |  |  |  |  |  | 2.40% | 2.20% | 0.00%
Scenario, Forecast [Member] | Other Post-retirement Benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |  |  |  |
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate | 5.00% |  |  |  |  |  |  |

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- Definition Minimum percentage increase in twelve-month average CPI-U necessary to receive COLA. + 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 The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512046 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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