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Investments in Marketable Securities - Schedule of Available-For-Sale and Trading Securities (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Available-for-Sale        
Cost $ 36.0   $ 66.1  
Unrealized gains 17.5   17.6  
Unrealized losses 0.0   (1.8)  
Fair Value 53.5   81.9  
Trading        
Cost     $ 43.2 $ 34.4
Unrealized gains 8.0 $ 6.6    
Unrealized losses (1.1) (0.5)    
Fair Value $ 50.1 $ 40.5