Debt and Other Obligations Debt and Other Obligations - General (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2013
Debt Instrument
Debt ceiling	$ 30,000
Face Amount	$ 40	$ 360
PARRS 1998 Series D Bond
Debt Instrument
PARRS interest rate after rate reset	2.134%
Amount of bonds redeemed	$ 318
Amount of redeemable bond issues outstanding	$ 256
PARRS interest rate after rate reset	2.134%
PARRS 1999 Series A Bond
Debt Instrument
PARRS interest rate after rate reset	2.216%
Amount of bonds redeemed	$ 316
Amount of redeemable bond issues outstanding	$ 208
PARRS interest rate after rate reset	2.216%