Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 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:
Derivative Liability, Statement Of Financial Position, Extensible Enumeration, Not Disclosed Flag	Derivatives	Derivatives
Contingent payment arrangements	$ 9,385	$ 252,690
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	146,781	146,906
Securities segregated (U.S. Treasury Bills)	499,245	867,679
Derivatives	20,941	14,518
Investments:
Total assets measured at fair value and other measurements	877,384	1,184,626
Derivatives	14,944	18,180
Contingent payment arrangements	9,385	252,690
Total liabilities measured at fair value	24,329	270,870
Recurring | Equity securities
Investments:
Investments	201,824	147,653
Recurring | Other investments
Investments:
Investments, other measurements		7,870
Total investments measured at fair value and other measurements	8,593
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	146,781	146,906
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	2,835	1
Investments:
Total assets measured at fair value	351,975	268,610
Derivatives	33	3,511
Contingent payment arrangements	0	0
Total liabilities measured at fair value	33	3,511
Level 1 | Recurring | Equity securities
Investments:
Investments	193,766	113,833
Level 1 | Recurring | Other investments
Investments:
Total investments	8,593	7,870
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	499,245	867,679
Derivatives	18,106	14,517
Investments:
Total assets measured at fair value	523,272	914,300
Derivatives	14,911	14,669
Contingent payment arrangements	0	0
Total liabilities measured at fair value	14,911	14,669
Level 2 | Recurring | Equity securities
Investments:
Investments	5,921	32,104
Level 2 | Recurring | Other investments
Investments:
Total investments	0	0
Level 3 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Investments:
Total assets measured at fair value	121	118
Derivatives	0	0
Contingent payment arrangements	9,385	252,690
Total liabilities measured at fair value	9,385	252,690
Level 3 | Recurring | Equity securities
Investments:
Investments	121	118
Level 3 | Recurring | Other investments
Investments:
Total investments	0	0
NAV Expedient | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Investments:
Total assets measured at fair value	2,016	1,598
Derivatives	0	0
Contingent payment arrangements	0	0
Total liabilities measured at fair value	0	0
NAV Expedient | Recurring | Equity securities
Investments:
Investments	2,016	1,598
NAV Expedient | Recurring | Other investments
Investments:
Total investments	$ 0	$ 0