Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Electric revenue	$ (13,483)	$ (12,123)	$ (11,888)
Pandemic Relief Credit	$ 225
Revenue from Contract with Customer [Abstract]
Percent of Pandemic Credit Offered	2.50%