Retirement and Post-Retirement Employee Benefit Plans - Postretirement Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Change in projected benefit obligation:
Plan amendments	$ (34,732)	$ 0
Postretirement Plans
Change in projected benefit obligation:
Benefit obligation at beginning of year	370,678	316,033
Service cost	11,000	13,466	$ 10,810
Interest cost	15,372	10,612	11,839
Plan participants’ contributions	5,648	5,849
Actuarial (gain) loss	6,800	43,412
Benefits paid	(19,610)	(18,694)
Benefit obligation at end of year	355,156	370,678	316,033
Change in plan assets:
Fair value of plan assets at beginning of year	208,245	201,901
Actual return on plan assets	53,335	2,356
Employer contributions	20,581	16,833
Plan participants’ contributions	5,648	5,849
Benefits paid	(19,610)	(18,694)
Fair value of plan assets at end of year	268,199	208,245	$ 201,901
Reconciliation:
Funded status	(86,957)	(162,433)
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (86,957)	$ (162,433)