Segment Reporting (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting
Number of Business Segment | segment									1
Net revenues	$ 129,640	$ 126,032	$ 127,474	$ 107,069	$ 122,313	$ 113,114	$ 110,206	$ 89,693	$ 490,215	$ 435,326	$ 395,209
Long-lived assets	169,966				179,400				169,966	179,400
High-temperature resistant alloys
Segment Reporting
Net revenues									392,172	352,614	320,119
Corrosive-resistant alloys
Segment Reporting
Net revenues									98,043	82,712	75,090
U.S. pension plan
Segment Reporting
Net revenues									300,728	258,275	235,500
Long-lived assets	163,158				172,689				163,158	172,689
Europe
Segment Reporting
Net revenues									119,246	113,967	98,096
Long-lived assets	6,661				6,522				6,661	6,522
China
Segment Reporting
Net revenues									24,329	24,640	18,997
Long-lived assets	$ 147				$ 189				147	189
Other
Segment Reporting
Net revenues									$ 45,912	$ 38,444	$ 42,616