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, 2024	Sep. 30, 2023	Sep. 30, 2022
Reconciliation:
Net amount recognized	$ 155,611	$ 92,648
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	71,003	75,687
Change in projected benefit obligation:
Benefit obligation at beginning of year	75,898	80,775
Service cost	55	845	$ 1,129
Interest cost	4,024	4,227	2,647
Actuarial loss	5,853	6
Benefits paid	(4,285)	(4,368)
Settlements	(9,601)	(5,587)
Benefit obligation at end of year	71,944	75,898	80,775
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	(71,944)	(75,898)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (71,944)	$ (75,898)