Summary of Significant Accounting Policies - Concentration (Details) - contract	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
U.S. Department of Defense
Risk Associated with Certain Concentrations
Number of contracts	2	2	2
U.S. Department of Defense | Revenue | Surplus Contract
Risk Associated with Certain Concentrations
Concentration risk (as a percent)	0.00%	12.40%	27.60%
U.S. Department of Defense | Revenue | Scrap Contract
Risk Associated with Certain Concentrations
Concentration risk (as a percent)	7.40%	10.20%	11.10%
Supplier Concentration Risk | Cost of Goods Sold | Contract with commercial client
Risk Associated with Certain Concentrations
Concentration risk (as a percent)	43.60%	33.70%	21.80%
Maximum
Risk Associated with Certain Concentrations
Term for release of funds to the seller	30 days