Retirement and Post-Retirement Employee Benefit Plans - Postretirement Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - Postretirement Plans - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Change in projected benefit obligation:
Benefit obligation at beginning of year	$ 274,098	$ 279,222
Service cost	12,078	12,436	$ 10,823
Interest cost	10,907	10,679	12,424
Plan participants’ contributions	4,720	4,936
Actuarial (gain) loss	(17,252)	(21,750)
Benefits paid	(18,565)	(13,970)
Plan amendments	0	2,545
Benefit obligation at end of year	265,986	274,098	279,222
Change in plan assets:
Fair value of plan assets at beginning of year	184,790	158,977
Actual return on plan assets	10,997	21,160
Employer contribution	17,419	13,687
Plan participants’ contributions	4,720	4,936
Benefits paid	(18,565)	(13,970)
Fair value of plan assets at end of year	199,361	184,790	$ 158,977
Reconciliation:
Funded status	(66,625)	(89,308)
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (66,625)	$ (89,308)