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, 2023	Sep. 30, 2022	Sep. 30, 2021
Reconciliation:
Net amount recognized	$ 92,648	$ 29,498
Postretirement Plan
Change in projected benefit obligation:
Benefit obligation at beginning of year	250,228	355,156
Service cost	6,183	10,235	$ 11,000
Interest cost	13,911	10,734	15,372
Plan participants’ contributions	2,053	3,210
Actuarial (gain) loss	(21,468)	(112,748)
Benefits paid	(16,903)	(16,359)
Benefit obligation at end of year	234,004	250,228	355,156
Change in plan assets:
Fair value of plan assets at beginning of year	229,686	268,199
Actual return on plan assets	27,833	(40,113)
Employer contributions	0	1,600
Benefits paid	(1,719)	0
Fair value of plan assets at end of year	255,800	229,686	$ 268,199
Reconciliation:
Funded status	21,796	(20,542)
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 21,796	$ (20,542)