Summary of Significant Accounting Policies Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Property, Plant and Equipment [Line Items]
Pre-commercial Ops - capitalized revenue	$ 2	$ 0	$ 2	$ (3)
Paradise CT Units 5-7
Property, Plant and Equipment [Line Items]
Pre-commercial Ops - capitalized revenue				3
Pre-commercial Ops - Capitalized Fuel				$ 3
Johnsonville Aeroderivative CT Units 25-28
Property, Plant and Equipment [Line Items]
Pre-commercial Ops - capitalized revenue	2
Pre-commercial Ops - Capitalized Fuel	$ 4		$ 5