Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Benefit Costs Postretirement (Details) - Postretirement Plans - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic postretirement cost:
Service cost	$ 10,810	$ 12,078	$ 12,436
Interest cost	11,839	10,907	10,679
Expected return on assets(1)	(10,659)	(8,006)	(7,185)
Amortization of transition asset	0	0	0
Amortization of prior service cost	173	11	(1,644)
Recognized actuarial loss(1)	(8,178)	(6,473)	(2,827)
Net periodic pension cost	$ 3,985	$ 8,517	$ 11,459