Postretirement Benefits - Change in Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Change in benefit obligations:
Actuarial (gain)	$ (101)
Amounts recognized in the consolidated balance sheets consist of:
Pension liabilities	(17,075)	$ (25,239)
Pension Plan
Change in benefit obligations:
Beginning balance, benefit obligations	128,852	133,152
Service cost	0	2,049	$ 1,894
Interest cost	4,098	3,793	4,143
Actuarial (gain)	(7,130)	(5,012)
Benefits and administrative expenses paid	(4,863)	(5,130)
Ending balance, benefit obligations	120,957	128,852	133,152
Change in plan assets:
Beginning balance, fair value of plan assets	103,613	97,980
Actual return on plan assets	4,701	10,388
Employer contributions	1,003	375
Benefits and administrative expenses paid	(4,863)	(5,130)
Ending balance, fair value of plan assets	104,454	103,613	$ 97,980
Amounts recognized in the consolidated balance sheets consist of:
Pension assets	572	0
Pension liabilities	(17,075)	(25,239)
Net amount recognized	(16,503)	(25,239)
Amounts recognized in accumulated other comprehensive loss (before income taxes) consist of:
Net actuarial loss	(12,344)	(18,103)
Total loss recognized	$ (12,344)	$ (18,103)
Weighted-average assumptions used to determine pension benefit obligations at year end:
Discount rate	4.20%	3.70%