Investments of Operating Entities and Consolidated Funds - Consolidated Funds - Securities Owned at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Securities owned, at fair value	$ 8,940	$ 10,622
Common Stock
Investment Holdings [Line Items]
Securities owned, at fair value	3,611	4,816
Warrants and Rights
Investment Holdings [Line Items]
Securities owned, at fair value	$ 5,329	$ 5,806