Retirement and Postretirement Employee Benefit Plans - 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
Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	412,160	428,629
Change in projected benefit obligation:
Benefit obligation at beginning of year	449,527	596,029
Service cost	10,805	16,165	$ 17,369
Interest cost	24,924	17,606	16,883
Actuarial gain	(16,085)	(141,567)
Benefits paid	(37,611)	(38,706)
Benefit obligation at end of year	431,560	449,527	596,029
Change in plan assets:
Fair value of plan assets at beginning of year	479,025	596,806
Actual return on plan assets	52,998	(87,575)
Employer contribution	8,000	8,500
Benefits paid	(37,611)	(38,706)
Fair value of plan assets at end of year	502,412	479,025	$ 596,806
Reconciliation:
Funded status	70,852	29,498
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 70,852	$ 29,498