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Fair Value Measurements - Estimated Values of Financial Instruments (Details) - USD ($)	Sep. 30, 2025	Sep. 30, 2024
Estimated Values of Financial Instruments (Level 2 Valuation)
EnergyRight receivables (including current portion)	$ 57,000,000	$ 56,000,000
Financing Receivable, after Allowance for Credit Loss	80,000,000	99,000,000
EnergyRight® financing obligations (including current portion)	74,000,000	74,000,000
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	18,000,000	19,000,000
Long-term outstanding power bonds (including current maturities), net	21,967,000,000	19,416,000,000
Long-term debt of variable interest entities (including current maturities)	1,696,000,000	966,000,000
Portion at Other than Fair Value Measurement [Member]
Estimated Values of Financial Instruments (Level 2 Valuation)
EnergyRight receivables (including current portion)	57,000,000	56,000,000
Reported Value Measurement
Estimated Values of Financial Instruments (Level 2 Valuation)
Financing Receivable, after Allowance for Credit Loss	86,000,000	105,000,000
EnergyRight® financing obligations (including current portion)	66,000,000	66,000,000
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	16,000,000	17,000,000
Long-term outstanding power bonds (including current maturities), net	21,831,000,000	18,889,000,000
Long-term debt of variable interest entities (including current maturities)	$ 1,681,000,000	$ 934,000,000

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