Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 47,565	$ 51,762
Contingent payment arrangements	$ 249,854	$ 247,309
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Brokers and dealers	Brokers and dealers
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 7,437	$ 7,750
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	37,122	42,526
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	3,006	1,486
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	151,698	95,521
Securities segregated (U.S. Treasury Bills)	878,235	1,521,705
Derivatives	18,261	24,712
Total investments measured at fair value and other measurements	212,521	217,518
Total assets measured at fair value and other measurements	1,260,715	1,859,456
Derivatives	14,665	13,712
Contingent payment arrangements	249,854	247,309
Total liabilities measured at fair value	264,519	261,021
Recurring | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	7,437	7,750
Recurring | Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	37,122	42,526
Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	157,663	159,067
Recurring | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments measured at fair value and other measurements	10,299	8,175
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	151,698	95,521
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,517	1,768
Total investments	135,099	136,344
Total assets measured at fair value	288,314	233,633
Derivatives	1,226	162
Contingent payment arrangements	0	0
Total liabilities measured at fair value	1,226	162
Level 1 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	127,806	129,655
Level 1 | Recurring | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	7,293	6,689
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	878,235	1,521,705
Derivatives	16,744	22,944
Total investments	28,217	27,799
Total assets measured at fair value	923,196	1,572,448
Derivatives	13,439	13,550
Contingent payment arrangements	0	0
Total liabilities measured at fair value	13,439	13,550
Level 2 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	28,217	27,799
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Total investments	121	129
Total assets measured at fair value	121	129
Derivatives	0	0
Contingent payment arrangements	249,854	247,309
Total liabilities measured at fair value	249,854	247,309
Level 3 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	121	129
NAV Expedient | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	1,519	1,484
Total assets measured at fair value	1,519	1,484
NAV Expedient | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 1,519	$ 1,484