Retirement and Post-Retirement Employee Benefit Plans - Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - Defined Benefit Plan - USD ($)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 558,639,000	$ 565,755,000
Change in projected benefit obligation:
Benefit obligation at beginning of year	604,221,000	577,270,000
Service cost	17,369,000	17,551,000	$ 15,311,000
Interest cost	16,883,000	19,028,000	22,071,000
Actuarial (gain) loss	(7,561,000)	22,898,000
Benefits paid	(34,883,000)	(32,526,000)
Benefit obligation at end of year	596,029,000	604,221,000	577,270,000
Change in plan assets:
Fair value of plan assets at beginning of year	528,881,000	530,109,000
Actual return on plan assets	92,808,000	31,298,000
Employer contribution	10,000,000	0
Benefits paid	(34,883,000)	(32,526,000)
Fair value of plan assets at end of year	596,806,000	528,881,000	$ 530,109,000
Reconciliation:
Funded status	777,000	(75,340,000)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 777,000	$ (75,340,000)