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CAPITAL AND FINANCIAL RISK MANAGEMENT - Liquidity risk (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt repayments      
Cash and cash equivalent balances $ 1,025.8 $ 827.0 $ 1,043.9
Liquidity risk      
Debt repayments      
Cash and cash equivalent balances 1,025.8    
Liquidity risk | Long-term debt      
Debt repayments      
Long-term debt 2,684.0    
Liquidity risk | 2018 | Long-term debt      
Debt repayments      
Long-term debt 95.6    
Liquidity risk | 2019, 2020 | Long-term debt      
Debt repayments      
Long-term debt 190.2    
Liquidity risk | 2021, 2022 | Long-term debt      
Debt repayments      
Long-term debt 664.5    
Liquidity risk | 2023 and thereafter | Long-term debt      
Debt repayments      
Long-term debt $ 1,733.7