Retirement and Postretirement Employee Benefit Plans - Postretirement 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
Postretirement Plan
Change in projected benefit obligation:
Benefit obligation at beginning of year	269,391	234,004
Service cost	8,132	6,028	$ 6,183
Interest cost	13,463	14,034	13,911
Plan participants’ contributions	1,993	2,102
Actuarial (gain) loss	(8,733)	31,135
Benefits paid	(19,065)	(17,912)
Plan amendments	1,873	0
Benefit obligation at end of year	267,054	269,391	234,004
Change in plan assets:
Fair value of plan assets at beginning of year	300,692	255,800
Actual return on plan assets	21,500	47,857
Employer contributions	0	0
Benefits paid	(2,808)	(2,965)
Fair value of plan assets at end of year	319,384	300,692	$ 255,800
Reconciliation:
Funded status	52,330	31,301
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 52,330	$ 31,301