Summary of Significant Accounting Policies - Reclassification Out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (1,222)	$ (211)
Net income (loss)	(743)	9,191
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	4,217	(808)
Amount reclassified from accumulated other comprehensive loss | Settlements/curtailments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	274	239
Amount reclassified from accumulated other comprehensive loss | Retirement Plan Liability Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before taxes	4,491	(569)
Income tax expense	0	0
Net income (loss)	$ 4,491	$ (569)