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Cash and cash equivalents (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Cash and cash equivalents.        
Cash deposits $ 62.3 $ 469.5    
Term deposits 9.8 41.6    
Cash and cash equivalents $ 72.1 $ 511.1 $ 614.3 $ 253.0