Income Taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Accrued employee benefits	$ 17,371	$ 11,409
Allowances for loss contingencies	2,726	3,561
Debt obligation basis difference	4,630
Deferred compensation	2,930	3,071
Foreign currency mark-to-market	919
Inventory valuation	4,897	8,036
Long-term contracts	7,335	6,995
Prepaid and accrued expenses	151	1,816
Property, plant and equipment	1,632
Retirement benefits	3,930	4,967
Tax credit carryforwards	39,356	33,118
Loss carryforwards	46,614	36,248
Other	308	818
Total gross deferred tax assets	132,799	110,039
Valuation allowance	(59,817)	(69,098)
Total deferred tax assets	72,982	40,941
Deferred tax liabilities:
Debt obligation basis difference		(4,582)
Deferred revenue	(15,617)	(12,135)
Intangible assets	(36,261)	(18,592)
Lease right-of-use assets	(20,271)
Property, plant and equipment		(4,524)
Unremitted foreign earnings	(2,159)	(977)
Other	(71)	(587)
Total deferred tax liabilities	(74,379)	(41,397)
Net deferred tax liabilities	$ (1,397)	$ (456)