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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]      
Investments in marketable securities $ 81.0 $ 77.8 $ 77.8
Proceeds received from sales of marketable securities 13.8 37.3  
Net realized gains on sale of marketable securities $ 2.8 $ 6.7