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	Sep. 30, 2022	Dec. 31, 2021
Securities and other investments owned:
Total securities and other investments owned	$ 1,238,613	$ 1,532,095
Securities sold not yet purchased:
Total securities sold not yet purchased	17,751	28,623
Corporate bonds
Securities and other investments owned:
Total securities and other investments owned	6,761	7,632
Securities sold not yet purchased:
Total securities sold not yet purchased	6,264	6,327
Other fixed income securities
Securities and other investments owned:
Total securities and other investments owned	8,649	2,606
Securities sold not yet purchased:
Total securities sold not yet purchased	686	1,994
Partnership interests and other
Securities and other investments owned:
Total securities and other investments owned	82,475	77,383
Equity securities
Securities and other investments owned:
Total securities and other investments owned	1,140,728	1,444,474
Securities sold not yet purchased:
Total securities sold not yet purchased	$ 10,801	$ 20,302