Fair Value Measurements Estimated Values of Financial Instruments Not Recorded at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Estimated Values of Financial Intruments Not Recorded at Fair Value
EnergyRight® receivables, net (including current portion)	$ 56	$ 56
Loans and other long-term receivables, net (including current portion)	108	99
EnergyRight® financing obligations (including current portion)	73	74
Membership interests of VIEs subject to mandatory redemption (including current portion)	18	19
Long-term outstanding power bonds, net (including current maturities)	20,483	19,416
Long-term debt of VIEs, net (including current maturities)	1,693	966
Reported Value Measurement
Estimated Values of Financial Intruments Not Recorded at Fair Value
EnergyRight® receivables, net (including current portion)	56	56
Loans and other long-term receivables, net (including current portion)	114	105
EnergyRight® financing obligations (including current portion)	65	66
Membership interests of VIEs subject to mandatory redemption (including current portion)	16	17
Long-term outstanding power bonds, net (including current maturities)	20,599	18,889
Long-term debt of VIEs, net (including current maturities)	$ 1,699	$ 934

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