Fair Value Measurements - Investments (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) Units	Sep. 30, 2024 USD ($)
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Long-term Investments	$ 5,573	$ 4,968
Period of time where the investor contributes capital to an investment in a private partnership - minimum \| Units	3
Period of time where the investor contributes capital to an investment in a private partnership - maximum \| Units	4
Minimum investment period	10 years
Fair value of gross plan assets	$ 8,837	8,959
Number of readily available quoted exchange prices for the investments	0
NDT
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Long-term Investments	$ 3,700
ART
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Long-term Investments	1,700
LTDCP
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gains (losses) on investments	1	2
SERP
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gains (losses) on investments	3	14
ART
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gains (losses) on investments	116	171
Debt and Equity Securities, Unrealized Gain (Loss)	7	36
NDT
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Unrealized gains (losses) on investments	221	324
Debt and Equity Securities, Unrealized Gain (Loss)	10	93
Equity Funds [Member] \| NDT
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	361
Real Estate Funds [Member] \| NDT
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	138
Credit [Member] \| NDT
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	117
Equity \| ART
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	145
Private real estate funds \| ART
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	89
Private Credit [Member] \| ART
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Commitments, Fair Value Disclosure	57
RP
Fair Value, Measured on Recurring Basis, Gain (Loss) Included in Earnings [Line Items]
Debt and Equity Securities, Unrealized Gain (Loss)	$ 1	$ 1

Tennessee Valley Authority 10-K