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Financial risk management objectives and policies - Past Due Receivables (Details) - Trade receivables - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of financial assets that are either past due or impaired [line items]    
Book value $ 54.0 $ 40.5
Neither past due nor impaired | Current    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value 37.3 33.4
Past due but not impaired | Less than 31 days    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value 11.0 5.5
Past due but not impaired | 31-60 days    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value 3.2 1.0
Past due but not impaired | 61-90 days    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value 1.7 0.5
Past due but not impaired | 91-121 days    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value 0.8 0.1
Past due but not impaired | Greater than 121 days    
Disclosure of financial assets that are either past due or impaired [line items]    
Book value $ 0.0 $ 0.0