SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of securities and other investments owned and securities sold not yet purchased at fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Securities and other investments owned:
Total securities and other investments owned	$ 1,144,896	$ 1,532,095
Securities sold not yet purchased:
Total securities sold not yet purchased	5,403	28,623
Corporate bonds
Securities and other investments owned:
Total securities and other investments owned	8,231	7,632
Securities sold not yet purchased:
Total securities sold not yet purchased	2,093	6,327
Other fixed income securities
Securities and other investments owned:
Total securities and other investments owned	2,321	2,606
Securities sold not yet purchased:
Total securities sold not yet purchased	3,084	1,994
Partnership interests and other
Securities and other investments owned:
Total securities and other investments owned	78,965	77,383
Equity securities
Securities and other investments owned:
Total securities and other investments owned	1,055,379	1,444,474
Securities sold not yet purchased:
Total securities sold not yet purchased	$ 226	$ 20,302