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Share capital - Share premium account (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Disclosure of classes of share capital [line items]        
Equity $ 162.3 $ 141.9 $ 169.7 $ 175.4
Share premium        
Disclosure of classes of share capital [line items]        
Equity $ 56.4 $ 56.4 $ 56.4 $ 56.2