Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Benefit Costs Postretirement (Details) - Postretirement Plans - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Components of net periodic postretirement cost:
Service cost	$ 12,078	$ 12,436	$ 10,823
Interest cost	10,907	10,679	12,424
Expected return on assets	(8,006)	(7,185)	(6,264)
Amortization of transition asset	0	0	82
Amortization of prior service cost	11	(1,644)	(1,644)
Recognized actuarial loss	(6,473)	(2,827)	(2,167)
Net periodic pension cost	$ 8,517	$ 11,459	$ 13,254