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, 2020	Sep. 30, 2019	Sep. 30, 2018
Change in projected benefit obligation:
Benefit obligation at beginning of year	$ 316,033	$ 265,986
Service cost	13,466	10,810	$ 12,078
Interest cost	10,612	11,839	10,907
Plan participants’ contributions	5,849	5,901
Actuarial (gain) loss	43,412	39,472
Benefits paid	(18,694)	(17,975)
Benefit obligation at end of year	370,678	316,033	265,986
Change in plan assets:
Fair value of plan assets at beginning of year	201,901	199,361
Actual return on plan assets	2,356	1,125
Employer contribution	16,833	13,489
Plan participants’ contributions	5,849	5,901
Benefits paid	(18,694)	(17,975)
Fair value of plan assets at end of year	208,245	201,901	$ 199,361
Reconciliation:
Funded status	(162,433)	(114,132)
Unrecognized prior service cost	0	0
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ (162,433)	$ (114,132)