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, 2022	Sep. 30, 2021	Sep. 30, 2020
Change in projected benefit obligation:
Plan amendments	$ 0	$ (34,732)
Reconciliation:
Net amount recognized	29,498	0
Postretirement Plan
Change in projected benefit obligation:
Benefit obligation at beginning of year	355,156	370,678
Service cost	10,235	11,000	$ 13,466
Interest cost	10,734	15,372	10,612
Plan participants’ contributions	3,210	5,648
Actuarial (gain) loss	(112,748)	6,800
Benefits paid	(16,359)	(19,610)
Benefit obligation at end of year	250,228	355,156	370,678
Change in plan assets:
Fair value of plan assets at beginning of year	268,199	208,245
Actual return on plan assets	(40,113)	53,335
Employer contributions	14,749	20,581
Plan participants’ contributions	3,210	5,648
Benefits paid	(16,359)	(19,610)
Fair value of plan assets at end of year	229,686	268,199	$ 208,245
Reconciliation:
Funded status	(20,542)	(86,957)
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (20,542)	$ (86,957)