Retirement Plans - Changes in the Fair Value of Pension Assets (Details) - Pension plan - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Pension
Fair value of plan assets at beginning of year	$ 293	$ 263
Fair value of plan assets at end of year	273	293
Significant Unobservable Inputs (Level 3)
Pension
Fair value of plan assets at beginning of year	0
Fair value of plan assets at end of year	0	0
Real estate | Significant Unobservable Inputs (Level 3)
Pension
Fair value of plan assets at beginning of year	0	4
Redemptions	0	(4)
Fair value of plan assets at end of year	$ 0	$ 0