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, 2023	Sep. 30, 2022	Sep. 30, 2021
Reconciliation:
Net amount recognized	$ 92,648	$ 29,498
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	75,687	79,233
Change in projected benefit obligation:
Benefit obligation at beginning of year	80,775	104,301
Service cost	845	1,129	$ 1,067
Interest cost	4,227	2,647	3,180
Actuarial (gain) loss	6	(22,471)
Benefits paid	(4,368)	(4,831)
Settlements	(5,587)	0
Benefit obligation at end of year	75,898	80,775	104,301
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	(75,898)	(80,775)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (75,898)	$ (80,775)