Summary of Significant Accounting Policies General and Basis of Presentation (Details) People in Millions, $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2020 USD ($) People	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) People	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Reclassification of Amounts on Cash Flow and Statement of Operations [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 800			$ 800		$ 299
Restricted Cash and Cash Equivalents	23			23		23
Regulatory Assets	9,400			9,400		8,919
Other Deferred Costs, Net	(10)			(10)
Appropriation-investment power program						1,000
Financing Receivable, after Allowance for Credit Loss	$ 122			$ 122		120
Population of TVA's service area | People	10			10
Allowance for uncollectible accounts	$ 1			$ 1		1
Depreciation expense	344		$ 536	1,300
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 823		$ 323	823	$ 323	$ 322	$ 322
Cash and Cash Equivalents, Period Increase (Decrease)		$ 500		$ 501	$ 1