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Benefit Plans Sensitivity to Certain Changes in Pension Assumptions (Details) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Oct. 01, 2023 | Oct. 01, 2016 Years | Sep. 30, 2016
Defined Benefit Plan Disclosure |  |  |  |  |
Defined Benefit Plan, Cost of Living Adjustment Assumption | 25.00% |  |  |  |
Discount rate |  | 3.70% |  |  |
Age Threshold for Changes to COLA Changes for Cash Balance Plan | Years |  |  |  | 50 |
Discount rate |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |
Change in Assumption | (0.25%) |  |  |  |
Impact on Pension Cost | $ 16,000,000 |  |  |  |
Impact on Projected Benefit Obligation | $ 330,000,000 |  |  |  |
Rate of return on plan assets |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |
Change in Assumption | (0.25%) |  |  |  |
Impact on Pension Cost | $ 18,000,000 |  |  |  |
Other Post-retirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |
Defined Benefit Plan, Cost of Living Adjustment Assumption | 2.00% | 2.00% |  |  | 2.40%
Discount rate | 4.40% | 3.95% |  |  |
Actuarial assumption COLA | $ 1.8400 | $ 0.9900 |  |  |
Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |
Defined Benefit Plan, Cost of Living Adjustment Assumption | 2.00% | 2.00% |  |  | 2.40%
COLA percentage increase (decrease) | 600.00% |  |  |  |
Discount rate | 4.35% | 3.85% |  |  |
Pre-Medicare Eligible [Member] | Other Post-retirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure |  |  |  |  |
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate | 5.00% | 5.00% |  |  | 5.00%
Scenario, Forecast | Pre-Medicare Eligible [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 Actuarial assumption COLA + References No definition available. + Details
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- Definition Age Threshold for Changes to COLA Related to 2016 Amendments to TVA Pension Plan + References No definition available. + Details
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- Definition COLA percentage increase (decrease) + References No definition available. + Details
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- Definition The change in the discount date or the rate of return on plan assets assumption that is used in disclosure of sensitivity of the net periodic pension cost and the projected benefit obligation to changes in those rates. + 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 Amount of effect of change in rate on the net periodic benefit cost for pension benefits. + References No definition available. + Details
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- Definition Amount of effect of change in rate on the projected benefit obligation for pension benefits. + 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=108410482&loc=d3e1928-114920 + Details
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- Definition Ultimate trend rate for health care cost for defined benefit postretirement 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 (l) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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