Retirement and Post-Retirement Employee Benefit Plans - Plans’ Accumulated Benefit Obligation, Projected Benefit Obligation and Funded Status (Details) - Defined Benefit Plan - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 478,750	$ 505,355
Change in projected benefit obligation:
Benefit obligation at beginning of year	533,455	545,480
Service cost	17,264	18,109	$ 16,419
Interest cost	20,803	20,443	23,193
Actuarial (gain) loss	(29,087)	(16,347)
Benefits paid	(37,716)	(34,230)
Benefit obligation at end of year	504,719	533,455	545,480
Change in plan assets:
Fair value of plan assets at beginning of year	508,244	473,950
Actual return on plan assets	54,163	63,524
Employer contribution	7,000	5,000
Benefits paid	(37,716)	(34,230)
Fair value of plan assets at end of year	531,691	508,244	$ 473,950
Reconciliation:
Funded status	26,972	(25,211)
Unrecognized prior service cost	0	0
Unrecognized net loss	0	0
Net amount recognized	$ 26,972	$ (25,211)