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Cash and cash equivalents (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Cash and Cash Equivalents.      
Cash deposits $ 60.3 $ 469.5  
Term deposits 9.4 41.6  
Cash and cash equivalents $ 69.7 $ 511.1 $ 253.0