Summary of Significant Accounting Policies - Pre-commercial Plant Ops (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue capitalized during pre-commercial operations	$ 4	$ 3	$ 3
Pre-commercial Operations - Capitalized Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Unregulated Operating Revenue	(4)	3	$ 3
Paradise CT Units 5-7
Revenues from External Customers and Long-Lived Assets [Line Items]
Fuel capitalized during pre-commercial ops		3
Paradise CT Units 5-7 | Pre-commercial Operations - Capitalized Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Unregulated Operating Revenue		$ 3
Johnsonville Aeroderivative CT Units 21-30
Revenues from External Customers and Long-Lived Assets [Line Items]
Fuel capitalized during pre-commercial ops	7
Johnsonville Aeroderivative CT Units 21-30 | Pre-commercial Operations - Capitalized Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Unregulated Operating Revenue	$ 4