Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Supplemental Executive Retirement Plans
Components of net periodic postretirement cost:
Service cost	$ 1,074	$ 869	$ 1,332
Interest cost	4,188	5,127	4,988
Recognized actuarial loss	3,945	2,227	3,079
Settlements	9,180	0	4,159
Net periodic pension cost	18,387	8,223	13,558
Defined Benefit Plan
Components of net periodic postretirement cost:
Service cost	17,551	15,311	17,264
Interest cost	19,028	22,071	20,803
Expected return on assets	(28,316)	(28,451)	(27,666)
Amortization of prior service cost	(231)	(232)	(231)
Recognized actuarial loss	9,025	4,201	9,114
Net periodic pension cost	$ 17,057	$ 12,900	$ 19,284