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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]          
Fair value $ 84.1   $ 84.1   $ 77.8
Proceeds received from sales of marketable securities 12.1 $ 21.0 25.9 $ 58.3  
Net realized gains on sale of marketable securities $ 1.3 $ 8.4 $ 4.1 $ 15.1