Summary of Significant Accounting Policies - Reclassification Out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	$ 8,980	$ (8,207)
Income tax expense	(211)	(701)
Net income (loss)	9,191	(7,506)
Amount reclassified from accumulated other comprehensive loss | Prior service costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	0	0
Amount reclassified from accumulated other comprehensive loss | Net actuarial gain (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	(808)	(4,214)
Amount reclassified from accumulated other comprehensive loss | Settlements/curtailments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	239	246
Amount reclassified from accumulated other comprehensive loss | Retirement Plan Liability Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	(569)	(3,968)
Income tax expense	0	0
Net income (loss)	$ (569)	$ (3,968)