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EXPENSES - Operating expenses by nature (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
EXPENSES    
Raw materials and consumables $ 126.6 $ 148.1
Salaries and employee benefits 112.5 102.1
Contractors 81.1 60.6
Repairs and maintenance 37.2 47.7
General and administrative 24.4 20.3
Leases 6.9 4.2
Royalties 6.2 3.5
Drilling and analytical 3.8 2.4
Other 6.1 5.2
Total production expenses 404.8 394.1
Less: Production expenses capitalized (71.6) (45.3)
Add (less): Change in inventories 38.7 (23.4)
Total operating expenses $ 371.9 $ 325.4