Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Assets, Fair Value Disclosure [Abstract]
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Brokers and dealers	Brokers and dealers
Investments:
Investment, Type [Extensible Enumeration]	Limited partnership hedge funds	Limited partnership hedge funds
Other	$ 38,188	$ 88,031
Contingent consideration liability	254,108	252,690
Time deposits
Investments:
Other	6,249	6,517
Limited partnership hedge funds
Investments:
Other	29,235	78,775
Other investments
Investments:
Other	2,704	2,739
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	155,596	146,906
Securities segregated (U.S. Treasury Bills)	865,253	867,679
Derivatives	13,857	14,518
Investments:
Other	29,235	78,775
Total investments measured at fair value and other measurements	220,268	243,554
Total assets measured at fair value and other measurements	1,254,974	1,272,657
Derivatives	11,859	18,180
Contingent consideration liability	254,108	252,690
Total liabilities measured at fair value	265,967	270,870
Recurring | Time deposits
Investments:
Other	6,249	6,517
Recurring | Equity securities
Investments:
Equity securities	173,644	147,653
Recurring | Other investments
Investments:
Total investments measured at fair value and other measurements	11,140	10,609
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	155,596	146,906
Derivatives	47	1
Investments:
Total investments	175,617	121,703
Total assets measured at fair value	331,260	268,610
Derivatives	751	3,511
Total liabilities measured at fair value	751	3,511
Level 1 | Recurring | Equity securities
Investments:
Equity securities	167,181	113,833
Level 1 | Recurring | Other investments
Investments:
Total investments	8,436	7,870
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Securities segregated (U.S. Treasury Bills)	865,253	867,679
Derivatives	13,810	14,517
Investments:
Total investments	6,319	32,104
Total assets measured at fair value	885,382	914,300
Derivatives	11,108	14,669
Total liabilities measured at fair value	11,108	14,669
Level 2 | Recurring | Equity securities
Investments:
Equity securities	6,319	32,104
Level 3 | Recurring
Investments:
Total investments	114	118
Total assets measured at fair value	114	118
Contingent consideration liability	254,108	252,690
Total liabilities measured at fair value	254,108	252,690
Level 3 | Recurring | Equity securities
Investments:
Equity securities	114	118
NAV Expedient | Recurring
Investments:
Total investments	30	1,598
Total assets measured at fair value	30	1,598
NAV Expedient | Recurring | Equity securities
Investments:
Equity securities	$ 30	$ 1,598