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Benefit Plans Components of Benefit Plans (Details) | 12 Months Ended | 240 Months Ended |
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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 |  |  |  |  |

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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 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). + References No definition available. + Details
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- Definition Funded Status Threshold to Reduce Minimum Contribution Under 2016 Changes to TVA Pension Plan + References No definition available. + Details
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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. + References No definition available. + Details
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- Definition Minimum Funding for TVA Pension Plan Under 2016 Changes + References No definition available. + Details
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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. + References No definition available. + Details
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- Definition Threshold for Deferral of Actuarial Gain/Loss Under Corridor Approach + References No definition available. + Details
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- Definition Years of Service Threshold Related to 2016 Amendments to TVA Pension Plan + References No definition available. + Details
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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. + References No definition available. + Details
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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 + Details
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