Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Assets, Fair Value Disclosure [Abstract]
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Brokers and dealers	Brokers and dealers
Investments:
Contingent consideration liability	$ 129,219	$ 252,690
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	148,676	146,906
Securities segregated (U.S. Treasury Bills)	545,928	867,679
Derivatives	13,038	14,518
Investments:
Total assets measured at fair value	915,328	1,184,626
Derivatives	17,908	18,180
Contingent consideration liability	129,219	252,690
Total liabilities measured at fair value	147,127	270,870
Recurring | Equity securities
Investments:
Equity securities	198,879	147,653
Recurring | Other investments
Investments:
Total investments measured at fair value and other measurements	8,807	7,870
Level 1 | Other investments
Investments:
Total investments	8,800	7,900
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	148,676	146,906
Derivatives	106	1
Investments:
Total assets measured at fair value	350,378	268,610
Derivatives	3,004	3,511
Total liabilities measured at fair value	3,004	3,511
Level 1 | Recurring | Equity securities
Investments:
Equity securities	192,789	113,833
Level 1 | Recurring | Other investments
Investments:
Total investments	8,807	7,870
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Securities segregated (U.S. Treasury Bills)	545,928	867,679
Derivatives	12,932	14,517
Investments:
Total assets measured at fair value	564,781	914,300
Derivatives	14,904	14,669
Total liabilities measured at fair value	14,904	14,669
Level 2 | Recurring | Equity securities
Investments:
Equity securities	5,921	32,104
Level 3 | Recurring
Investments:
Total assets measured at fair value	138	118
Contingent consideration liability	129,219	252,690
Total liabilities measured at fair value	129,219	252,690
Level 3 | Recurring | Equity securities
Investments:
Equity securities	138	118
NAV Expedient | Recurring
Investments:
Total assets measured at fair value	31	1,598
NAV Expedient | Recurring | Equity securities
Investments:
Equity securities	$ 31	$ 1,598