Postretirement Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 270	$ 548	$ 482
Interest cost	27	71	124
Amortization of losses	11	(159)	(210)
Expected return on assets	(134)	2	2
Net periodic pension cost	174	462	398
Curtailment gain		(227)
Settlement (gain) loss	(259)		232
Total pension cost (gain)	$ (85)	$ 235	$ 630