SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of financial assets and liabilities measured on recurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets:
Funds held in trust account for BRPM 250 to redeem noncontrolling interests in equity of subsidiaries	$ 0	$ 174,437
Securities and other investments owned:
Total securities and other investments owned	1,079,097	965,096
Loans receivable, at fair value	549,142	701,652
Total assets measured at fair value	1,628,239	1,841,185
Securities sold not yet purchased:
Total securities sold not yet purchased	7,120	5,897
Mandatorily redeemable noncontrolling interests issued after November 5, 2003	4,584	4,648
Warrant liabilities		173
Contingent consideration	27,987	31,046
Total liabilities measured at fair value	39,691	41,764
Quoted prices in active markets for identical assets (Level 1)
Assets:
Funds held in trust account for BRPM 250 to redeem noncontrolling interests in equity of subsidiaries		174,437
Securities and other investments owned:
Total securities and other investments owned	390,548	584,136
Loans receivable, at fair value	0	0
Total assets measured at fair value	390,548	758,573
Securities sold not yet purchased:
Total securities sold not yet purchased	3,561	4,466
Mandatorily redeemable noncontrolling interests issued after November 5, 2003	0	0
Warrant liabilities		173
Contingent consideration	0	0
Total liabilities measured at fair value	3,561	4,639
Other observable inputs (Level 2)
Assets:
Funds held in trust account for BRPM 250 to redeem noncontrolling interests in equity of subsidiaries		0
Securities and other investments owned:
Total securities and other investments owned	13,432	12,495
Loans receivable, at fair value	0	0
Total assets measured at fair value	13,432	12,495
Securities sold not yet purchased:
Total securities sold not yet purchased	3,559	1,431
Mandatorily redeemable noncontrolling interests issued after November 5, 2003	0	0
Warrant liabilities		0
Contingent consideration	0	0
Total liabilities measured at fair value	3,559	1,431
Significant unobservable inputs (Level 3)
Assets:
Funds held in trust account for BRPM 250 to redeem noncontrolling interests in equity of subsidiaries		0
Securities and other investments owned:
Total securities and other investments owned	675,117	368,465
Loans receivable, at fair value	549,142	701,652
Total assets measured at fair value	1,224,259	1,070,117
Securities sold not yet purchased:
Total securities sold not yet purchased	0	0
Mandatorily redeemable noncontrolling interests issued after November 5, 2003	4,584	4,648
Warrant liabilities		0
Contingent consideration	27,987	31,046
Total liabilities measured at fair value	32,571	35,694
Equity securities
Securities and other investments owned:
Total securities and other investments owned	1,008,884	952,601
Securities sold not yet purchased:
Total securities sold not yet purchased	458	4,466
Equity securities | Quoted prices in active markets for identical assets (Level 1)
Securities and other investments owned:
Total securities and other investments owned	333,767	584,136
Securities sold not yet purchased:
Total securities sold not yet purchased	458	4,466
Equity securities | Other observable inputs (Level 2)
Securities and other investments owned:
Total securities and other investments owned	0	0
Securities sold not yet purchased:
Total securities sold not yet purchased	0	0
Equity securities | Significant unobservable inputs (Level 3)
Securities and other investments owned:
Total securities and other investments owned	675,117	368,465
Securities sold not yet purchased:
Total securities sold not yet purchased	0	0
Corporate bonds
Securities and other investments owned:
Total securities and other investments owned	66,341	8,539
Securities sold not yet purchased:
Total securities sold not yet purchased	4,057	1,162
Corporate bonds | Quoted prices in active markets for identical assets (Level 1)
Securities and other investments owned:
Total securities and other investments owned	56,781	0
Securities sold not yet purchased:
Total securities sold not yet purchased	3,103	0
Corporate bonds | Other observable inputs (Level 2)
Securities and other investments owned:
Total securities and other investments owned	9,560	8,539
Securities sold not yet purchased:
Total securities sold not yet purchased	954	1,162
Corporate bonds | Significant unobservable inputs (Level 3)
Securities and other investments owned:
Total securities and other investments owned	0	0
Securities sold not yet purchased:
Total securities sold not yet purchased	0	0
Other fixed income securities
Securities and other investments owned:
Total securities and other investments owned	3,872	3,956
Securities sold not yet purchased:
Total securities sold not yet purchased	2,605	269
Other fixed income securities | Quoted prices in active markets for identical assets (Level 1)
Securities and other investments owned:
Total securities and other investments owned	0	0
Securities sold not yet purchased:
Total securities sold not yet purchased	0	0
Other fixed income securities | Other observable inputs (Level 2)
Securities and other investments owned:
Total securities and other investments owned	3,872	3,956
Securities sold not yet purchased:
Total securities sold not yet purchased	2,605	269
Other fixed income securities | Significant unobservable inputs (Level 3)
Securities and other investments owned:
Total securities and other investments owned	0	0
Securities sold not yet purchased:
Total securities sold not yet purchased	$ 0	$ 0