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Revenues from Contracts with Customers - Other Information (Details)
$ in Thousands, lb in Millions
6 Months Ended
Nov. 17, 2006
USD ($)
Mar. 31, 2019
USD ($)
lb
Sep. 30, 2018
USD ($)
Revenues from Contracts with Customers      
Accounts receivable, gross   $ 82,489 $ 74,567
Accounts receivable, allowance for doubtful accounts   1,401 1,130
Contract liabilities   $ 19,079 $ 20,329
Revenue, Remaining Performance Obligation, Optional Exemption, Performance Obligation   true  
Conversion Services Agreement      
Revenues from Contracts with Customers      
Revenue recognition period 20 years 20 years  
Advance payments received $ 50,000    
Conversion Services Agreement | Maximum      
Revenues from Contracts with Customers      
Annual volume of titanium metal to be converted (in pounds) | lb   10