Postretirement Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 599	$ 382	$ 268
Interest cost	118	75	22
Amortization of losses	(18)	5	7
Expected return on assets	(74)	(66)	(130)
Net periodic pension cost	625	396	167
Settlement gain			(259)
Total pension cost (gain)	$ 625	$ 396	$ (92)