Retirement and Post-Retirement Employee Benefit Plans - Supplemental Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - Supplemental Executive Retirement Plans - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 100,981	$ 122,207
Change in projected benefit obligation:
Benefit obligation at beginning of year	129,140	143,987
Service cost	1,067	1,074	$ 869
Interest cost	3,180	4,188	5,127
Actuarial (gain) loss	1,332	7,386
Benefits paid	(4,720)	(4,766)
Settlements	(25,698)	(22,729)
Benefit obligation at end of year	104,301	129,140	143,987
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Employer contribution	30,418	27,495
Benefits paid	(4,720)	(4,766)
Settlements	(25,698)	(22,729)
Fair value of plan assets at end of year	0	0	$ 0
Reconciliation:
Funded status	(104,301)	(129,140)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (104,301)	$ (129,140)