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Commitments and Contingencies - Legal Proceedings (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($) Units	Sep. 30, 2024 USD ($)
Legal Proceedings
Contribution related to TVARS case	$ 5	$ 5
Amount remaining to be spent under environmental agreements	$ 7
Megawatts of Cumberland Combined Cycle Plant [Abstract] | Units	1,450
General
Legal Proceedings
Legal loss contingency accrual	$ 11
Environmental Agreements
Legal Proceedings
Amount to be invested in certain environmental projects	290
Amount invested in certain environmental projects	285
Other long-term liabilities | General
Legal Proceedings
Legal loss contingency accrual	9
Accounts payable and accrued liabilities | General
Legal Proceedings
Legal loss contingency accrual	$ 2

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