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Investments in Marketable Securities - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Mar. 31, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]          
Proceeds received from sales of marketable securities $ 2.9   $ 16.6 $ 21.4 $ 42.6
Net realized gains (losses) on sale of marketable securities         $ 4.0
Affiliate sponsored investment products          
Investment [Line Items]          
Fair value of deconsolidated investment in affiliate   $ 84.3