Leases - Quantitative information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Option to renew	true
Weighted average remaining lease term for operating leases	10 months 17 days
Weighted average discount rate used to measure operating lease liabilities	4.48%
Operating lease, existence of material residual value guarantees	false
Inventory
Lessee, Lease, Description [Line Items]
Capitalized variable lease costs	$ 2.4
Property, plant and mine development
Lessee, Lease, Description [Line Items]
Capitalized variable lease costs	$ 4.7
Maximum
Lessee, Lease, Description [Line Items]
Renewal term	2 years
Minimum
Lessee, Lease, Description [Line Items]
Renewal term	1 year