Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Supplemental Executive Retirement Plans
Components of net periodic postretirement cost:
Service cost	$ 869	$ 1,332	$ 2,756
Interest cost	5,127	4,988	4,744
Recognized actuarial loss(1)	2,227	3,079	4,251
Settlements(1)	0	4,159	2,685
Net periodic pension cost	8,223	13,558	14,436
Defined Benefit Plan
Components of net periodic postretirement cost:
Service cost	15,311	17,264	18,109
Interest cost	22,071	20,803	20,443
Expected return on assets(1)	(28,451)	(27,666)	(27,975)
Amortization of prior service cost	(232)	(231)	(231)
Recognized actuarial loss(1)	4,201	9,114	12,744
Net periodic pension cost	$ 12,900	$ 19,284	$ 23,090