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Investments in Marketable Securities - Summary of the Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments (Details) - Equity Securities - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Available-for-Sale    
Cost $ 66.1 $ 104.7
Unrealized Gains 17.6 77.6
Unrealized Losses (1.8) (1.8)
Fair Value 81.9 180.5
Trading    
Cost 34.4 19.8
Unrealized Gains 6.6 1.9
Unrealized Losses (0.5) (2.3)
Fair Value $ 40.5 $ 19.4