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Expenses - Operating expenses by nature (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
EXPENSES    
Raw materials and consumables $ 131.1 $ 126.6
Salaries and employee benefits 117.0 112.5
Contractors 64.2 81.1
Repairs and maintenance 37.0 37.2
General and administrative 23.0 24.4
Leases 4.1 6.9
Royalties 8.5 6.2
Drilling and analytical 3.8 3.8
Other 13.3 6.1
Total production expenses 402.0 404.8
Less: Production expenses capitalized (70.9) (71.6)
Add (less): Change in inventories 8.8 38.7
Total operating expenses 339.9 $ 371.9
Reduction to salaries and employee benefits $ 11.6