Retirement and Post-Retirement Employee Benefit Plans - Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - Defined Benefit Plan - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 565,755	$ 541,287
Change in projected benefit obligation:
Benefit obligation at beginning of year	577,270	504,719
Service cost	17,551	15,311	$ 17,264
Interest cost	19,028	22,071	20,803
Actuarial (gain) loss	22,898	71,139
Benefits paid	(32,526)	(35,970)
Benefit obligation at end of year	604,221	577,270	504,719
Change in plan assets:
Fair value of plan assets at beginning of year	530,109	531,691
Actual return on plan assets	31,298	25,888
Employer contribution	0	8,500
Benefits paid	(32,526)	(35,970)
Fair value of plan assets at end of year	528,881	530,109	$ 531,691
Reconciliation:
Funded status	(75,340)	(47,161)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (75,340)	$ (47,161)