Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	Jun. 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	$ 255,166	$ 252,690
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	163,955	146,906
Securities segregated (U.S. Treasury Bills)	591,569	867,679
Derivatives	15,339	14,518
Investments:
Total assets measured at fair value	963,041	1,184,626
Derivatives	10,964	18,180
Contingent consideration liability	255,166	252,690
Total liabilities measured at fair value	266,130	270,870
Recurring | Equity securities
Investments:
Equity securities	183,886	147,653
Recurring | Other investments
Investments:
Total investments measured at fair value and other measurements	8,292	7,870
Level 1 | Other investments
Investments:
Total investments	8,300	7,900
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	163,955	146,906
Derivatives	32	1
Investments:
Total assets measured at fair value	351,324	268,610
Derivatives	556	3,511
Total liabilities measured at fair value	556	3,511
Level 1 | Recurring | Equity securities
Investments:
Equity securities	179,045	113,833
Level 1 | Recurring | Other investments
Investments:
Total investments	8,292	7,870
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Securities segregated (U.S. Treasury Bills)	591,569	867,679
Derivatives	15,307	14,517
Investments:
Total assets measured at fair value	611,571	914,300
Derivatives	10,408	14,669
Total liabilities measured at fair value	10,408	14,669
Level 2 | Recurring | Equity securities
Investments:
Equity securities	4,695	32,104
Level 3 | Recurring
Investments:
Total assets measured at fair value	116	118
Contingent consideration liability	255,166	252,690
Total liabilities measured at fair value	255,166	252,690
Level 3 | Recurring | Equity securities
Investments:
Equity securities	116	118
NAV Expedient | Recurring
Investments:
Total assets measured at fair value	30	1,598
NAV Expedient | Recurring | Equity securities
Investments:
Equity securities	$ 30	$ 1,598