Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	Dec. 31, 2020 USD ($)
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	$ 25.5
Concentrate manufacturing
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	23.8
Ancillary supplies
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	$ 1.7