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CAPITAL AND FINANCIAL RISK MANAGEMENT - Currency risk management (Details) - Currency risk
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
Capital and financial risk management  
Percentage of strengthening in U.S. dollar 10.00%
Percentage of weakening in U.S. dollar 10.00%
Foreign currency forward and collar contracts  
Capital and financial risk management  
Percentage of strengthening in U.S. dollar 10.00%
Percentage of weakening in U.S. dollar 10.00%
Canadian dollar  
Capital and financial risk management  
Foreign currency net working capital $ (26.3)
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) 2.4
Weakening in U.S dollar effect on earnings before taxes, gain (loss) (2.9)
Brazilian real  
Capital and financial risk management  
Foreign currency net working capital (91.3)
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) 8.3
Weakening in U.S dollar effect on earnings before taxes, gain (loss) (10.1)
Chilean pesos  
Capital and financial risk management  
Foreign currency net working capital (5.0)
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) 0.5
Weakening in U.S dollar effect on earnings before taxes, gain (loss) (0.6)
Russian rouble  
Capital and financial risk management  
Foreign currency net working capital 33.4
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) (3.0)
Weakening in U.S dollar effect on earnings before taxes, gain (loss) 3.7
Mauritanian ouguiya  
Capital and financial risk management  
Foreign currency net working capital (68.1)
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) 6.2
Weakening in U.S dollar effect on earnings before taxes, gain (loss) (7.6)
Other  
Capital and financial risk management  
Foreign currency net working capital 7.0
Strengthening in U.S dollar effect on earnings before taxes, gain (loss) (0.6)
Weakening in U.S dollar effect on earnings before taxes, gain (loss) $ 0.8