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, 2025	Sep. 30, 2024	Sep. 30, 2023
Reconciliation:
Net amount recognized	$ 226,409	$ 155,611
Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	428,845	448,353
Change in projected benefit obligation:
Benefit obligation at beginning of year	470,902	431,560
Service cost	10,047	9,546	$ 10,805
Interest cost	23,164	25,731	24,924
Actuarial (gain) loss	(18,133)	44,205
Benefits paid	(35,556)	(40,140)
Benefit obligation at end of year	450,424	470,902	431,560
Change in plan assets:
Fair value of plan assets at beginning of year	595,212	502,412
Actual return on plan assets	64,847	127,940
Employer contribution	0	5,000
Benefits paid	(35,556)	(40,140)
Fair value of plan assets at end of year	624,503	595,212	$ 502,412
Reconciliation:
Funded status	174,079	124,310
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 174,079	$ 124,310