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Cash and Cash Equivalents (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and Cash Equivalents.      
Cash deposits $ 469.5 $ 182.4  
Term deposits 41.6 70.6  
Cash and cash equivalents $ 511.1 $ 253.0 $ 149.2