Commitments and Contingencies - Purchase Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Recorded Unconditional Purchase Obligation [Line Items]
Purchase Obligation	$ 25.7
Cash collateral and escrow held by insurance carrier for workers' compensation insurance		$ 0.3
Concentrate manufacturing
Recorded Unconditional Purchase Obligation [Line Items]
Purchase Obligation	24.5
Ancillary supplies
Recorded Unconditional Purchase Obligation [Line Items]
Purchase Obligation	$ 1.2