Other Long-Term Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Assets, Noncurrent [Abstract]
Financing Receivable, after Allowance for Credit Loss, Noncurrent	$ 83	$ 84
Financing Receivable, after Allowance for Credit Loss, Current	3	21
Prepaid capital assets	81	29
Total other long-term assets	506	344
Other Long-Term Assets
Other Assets, Miscellaneous, Current	12	7
Other Intangible Assets, Net	114	35
Prepaid Expense, Current	7	6
Prepaid long-term service agreements, current	16	7
Transfers Accounted for as Secured Borrowings, Assets, Carrying Amount	57	56
Prepaid Expense and Other Assets, Noncurrent	89	62
Prepaid capital assets	81	29
Accounts and Financing Receivable, Allowance for Credit Loss	2	2
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization	14	15	$ 7
Accounts Receivable [Member]
Other Long-Term Assets
Transfers Accounted for as Secured Borrowings, Assets, Carrying Amount	12	12
Other Noncurrent Assets [Member]
Assets, Noncurrent [Abstract]
Other Assets, Miscellaneous, Noncurrent	84	88
Other Long-Term Assets
Other Assets, Miscellaneous, Noncurrent	84	88
Transfers Accounted for as Secured Borrowings, Assets, Carrying Amount	45	44
Other current assets
Other Long-Term Assets
Other Intangible Assets, Net	3	13
EnergyRight loan reserve
Other Long-Term Assets
Accounts and Financing Receivable, Allowance for Credit Loss	1	1
Economic development loan specific loan reserve
Other Long-Term Assets
Accounts and Financing Receivable, Allowance for Credit Loss	1	1
Other Noncurrent Assets [Member]
Assets, Noncurrent [Abstract]
Commodity contract derivative assets	10	2
Other Long-Term Assets
Commodity contract derivative assets	$ 10	$ 2
Energy Right
Other Long-Term Assets
Number of days in default	180 days
Minimum | Energy Right
Other Long-Term Assets
EnergyRight loan terms	5 years
Maximum | Energy Right
Other Long-Term Assets
EnergyRight loan terms	10 years