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CAPITAL AND FINANCIAL RISK MANAGEMENT - Energy price risk (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Capital and financial risk management  
Percentage of increase in price 10.00%
Percentage of decrease in price 10.00%
10% increase in price | WTI oil  
Capital and financial risk management  
Increase in price effect on OCI before tax, gain (loss) $ 9.0
10% decrease in price | WTI oil  
Capital and financial risk management  
Decrease in price effect on OCI before tax, gain (loss) $ (8.9)