Derivative Financial Instruments - Narrative (Details) $ in Thousands	3 Months Ended			8 Months Ended	9 Months Ended
	Dec. 31, 2019 oz	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Aug. 31, 2020 oz	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Fixed Interest Rate Swap One
Derivative [Line Items]
Derivative, Notional Amount		$ 50,000			$ 50,000
Concentrate Sales Contracts
Derivative [Line Items]
Provisional gain (loss) on derivatives and commodity contracts		233	$ 34		(1,461)	$ 15
Zinc options
Derivative [Line Items]
Net derivative gain (loss)		0	225		0
Interest rate swap
Derivative [Line Items]
Derivative, Notional Amount		37,500			37,500
Net derivative gain (loss)		15	$ 206		(173)	$ 18
Fixed Interest Rate Swap Two
Derivative [Line Items]
Derivative, Notional Amount		75,000			75,000
Derivative Instruments Settle in Year One [Member] | Fixed Interest Rate Swap Two
Derivative [Line Items]
Derivative, Notional Amount		$ 37,500			37,500
Designated as Hedging Instrument | Gold zero cost collars
Derivative [Line Items]
After tax gains in AOCI					$ 1,100
Designated as Hedging Instrument | Gold zero cost collars | Forecast
Derivative [Line Items]
Ounces of gold per month (in ounces) | oz	21,000			12,000