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Leases - Quantitative information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Lessee, Lease, Description [Line Items]        
Option to renew     true  
Weighted average remaining lease term for operating leases 3 months 7 days   3 months 7 days  
Weighted average discount rate used to measure operating lease liabilities 4.48%   4.48%  
Operating lease, existence of material residual value guarantees     false  
Inventory        
Lessee, Lease, Description [Line Items]        
Capitalized variable lease costs $ 1.8 $ 0.3 $ 4.6 $ 1.8
Property, plant and mine development        
Lessee, Lease, Description [Line Items]        
Capitalized variable lease costs   $ 1.4 $ 1.5 $ 3.2
Maximum        
Lessee, Lease, Description [Line Items]        
Renewal term 2 years   2 years  
Minimum        
Lessee, Lease, Description [Line Items]        
Renewal term 1 year   1 year