Retirement and Postretirement Employee Benefit Plans - Supplemental Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Reconciliation:
Net amount recognized	$ 226,409	$ 155,611
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	73,570	71,003
Change in projected benefit obligation:
Benefit obligation at beginning of year	71,944	75,898
Service cost	1,302	55	$ 845
Interest cost	3,488	4,024	4,227
Actuarial loss	2,547	5,853
Benefits paid	(4,761)	(4,285)
Settlements	0	(9,601)
Benefit obligation at end of year	74,520	71,944	75,898
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Employer contribution	0	0
Benefits paid	0	0
Settlements	0	0
Fair value of plan assets at end of year	0	0	$ 0
Reconciliation:
Funded status	(74,520)	(71,944)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (74,520)	$ (71,944)