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Financial instruments - Derivative Financial Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of financial liabilities [line items]    
Book value $ (178.4) $ (176.0)
Forward foreign currency exchange rate contracts    
Disclosure of financial liabilities [line items]    
Book value (0.7) (3.1)
Fair value (0.7) (3.1)
LME derivative contracts    
Disclosure of financial liabilities [line items]    
Book value 1.2 (0.6)
Fair value $ 1.2 $ (0.6)