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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]          
Proceeds received from sales of marketable securities $ 3.4   $ 12.1 $ 18.5 $ 25.9
Net realized gains (losses) on sale of marketable securities $ 0.2   $ 1.3 $ 0.7 $ 4.1
Affiliate sponsored investment products          
Investment [Line Items]          
Fair value of deconsolidated investment in affiliate   $ 84.3