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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - General (Details)
People in Millions, $ in Millions
 12 Months Ended
Sep. 30, 2015
USD ($)
People
Customers
Population of Service Area  
Population of TVA's service area (number of people) | People 9
Maximum original maturity 3 months
Number of customers for whom unbilled revenues are estimated | Customers 6
Recorded cost for emission allowances granted by the Environmental Protection Agency $ 0
Possible amount of future payments to the Department of Energy under the blended low-enriched uranium program 162
Payments attributable to blended low-enriched uranium fuel currently in use 131
BLEU fuel obligation 12
Amount of appropriation investment that must be repaid $ 1,000