Commitments and Contingencies - Purchase Obligations (Details) $ in Millions	Dec. 31, 2019 USD ($)
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	$ 24.9
Concentrate manufacturing
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	23.7
Ancillary supplies
Recorded Unconditional Purchase Obligation [Line Items]
Purchase obligation	$ 1.2