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FINANCIAL RISK MANAGEMENT - Metal and input price risk (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Gold    
FINANCIAL RISK MANAGEMENT    
Impact on net earnings due to 10 % change in price $ 19.0 $ 37.6
Impact on other comprehensive income due to 10 % change in price 0.0 0.0
Copper    
FINANCIAL RISK MANAGEMENT    
Impact on net earnings due to 10 % change in price 20.7 6.5
Impact on other comprehensive income due to 10 % change in price 0.0 0.0
Fuels and electricity    
FINANCIAL RISK MANAGEMENT    
Impact on net earnings due to 10 % change in price 7.0 $ 5.5
Impact on other comprehensive income due to 10 % change in price $ 0.0