Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Benefit Costs Postretirement (Details) - Postretirement Plans - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Components of net periodic postretirement cost:
Service cost	$ 13,466	$ 10,810	$ 12,078
Interest cost	10,612	11,839	10,907
Expected return on assets	(10,499)	(10,659)	(8,006)
Amortization of transition obligation	0	0	0
Amortization of prior service cost	173	173	11
Recognized actuarial loss	(1,337)	(8,178)	(6,473)
Net periodic pension cost	$ 12,415	$ 3,985	$ 8,517