Investments of Operating Entities and Consolidated Funds - Consolidated Funds - Securities Owned at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Securities owned, at fair value	$ 187,991	$ 375,278
Preferred Stock
Investment Holdings [Line Items]
Securities owned, at fair value	0	4,393
Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value	106,549	200,306
Government Securities
Investment Holdings [Line Items]
Securities owned, at fair value	0	161,607
Corporate Bonds
Investment Holdings [Line Items]
Securities owned, at fair value	0	3,405
Warrants and Rights [Member]
Investment Holdings [Line Items]
Securities owned, at fair value	5,904	5,567
Convertible Bonds
Investment Holdings [Line Items]
Securities owned, at fair value	$ 75,538	$ 0