Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets, Fair Value Disclosure [Abstract]
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Brokers and dealers	Brokers and dealers
Investments:
Investment, Type [Extensible Enumeration]	Hedge Funds	Hedge Funds
Investments, other measurements	$ 88,031	$ 51,762
Contingent payment arrangements	252,690	247,309
Time deposits
Investments:
Investments, other measurements	6,517	7,750
Hedge Funds
Investments:
Investments, other measurements	78,775	42,526
Other investments
Investments:
Investments, other measurements	2,739	1,486
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	146,906	95,521
Securities segregated (U.S. Treasury Bills)	867,679	1,521,705
Derivatives	14,518	24,712
Investments:
Investments, other measurements	78,775	42,526
Total investments measured at fair value and other measurements	243,554	217,518
Total assets measured at fair value and other measurements	1,272,657	1,859,456
Derivatives	18,180	13,712
Contingent payment arrangements	252,690	247,309
Total liabilities measured at fair value	270,870	261,021
Recurring | Time deposits
Investments:
Investments, other measurements	6,517	7,750
Recurring | Equity securities
Investments:
Investments	147,653	159,067
Recurring | Other investments
Investments:
Investments, other measurements		8,175
Total investments measured at fair value and other measurements	10,609
Level 1 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	146,906	95,521
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1	1,768
Investments:
Total investments	121,703	136,344
Total assets measured at fair value	268,610	233,633
Derivatives	3,511	162
Contingent payment arrangements	0	0
Total liabilities measured at fair value	3,511	162
Level 1 | Recurring | Equity securities
Investments:
Investments	113,833	129,655
Level 1 | Recurring | Other investments
Investments:
Total investments	7,870	6,689
Level 2 | Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	867,679	1,521,705
Derivatives	14,517	22,944
Investments:
Total investments	32,104	27,799
Total assets measured at fair value	914,300	1,572,448
Derivatives	14,669	13,550
Contingent payment arrangements	0	0
Total liabilities measured at fair value	14,669	13,550
Level 2 | Recurring | Equity securities
Investments:
Investments	32,104	27,799
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 investments	118	129
Total assets measured at fair value	118	129
Derivatives	0	0
Contingent payment arrangements	252,690	247,309
Total liabilities measured at fair value	252,690	247,309
Level 3 | Recurring | Equity securities
Investments:
Investments	118	129
Level 3 | Recurring | Other investments
Investments:
Total investments	0	0
NAV Expedient | Recurring
Investments:
Total investments	1,598	1,484
Total assets measured at fair value	1,598	1,484
NAV Expedient | Recurring | Equity securities
Investments:
Investments	$ 1,598	$ 1,484