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, 2024	Sep. 30, 2023	Sep. 30, 2022
Reconciliation:
Net amount recognized	$ 155,611	$ 92,648
Postretirement Plan
Change in projected benefit obligation:
Benefit obligation at beginning of year	234,004	250,228
Service cost	6,028	6,183	$ 10,235
Interest cost	14,034	13,911	10,734
Plan participants’ contributions	2,102	2,053
Actuarial (gain) loss	31,135	(21,468)
Benefits paid	(17,912)	(16,903)
Benefit obligation at end of year	269,391	234,004	250,228
Change in plan assets:
Fair value of plan assets at beginning of year	255,800	229,686
Actual return on plan assets	47,857	27,833
Employer contributions	0	0
Benefits paid	(2,965)	(1,719)
Fair value of plan assets at end of year	300,692	255,800	$ 229,686
Reconciliation:
Funded status	31,301	21,796
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 31,301	$ 21,796