Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 87,667	$ 51,762
Contingent payment arrangements	$ 251,522	$ 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	$ 7,262	$ 7,750
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	77,433	42,526
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	2,972	1,486
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	171,192	95,521
Securities segregated (U.S. Treasury Bills)	927,934	1,521,705
Derivatives	24,428	24,712
Total investments measured at fair value and other measurements	238,214	217,518
Total assets measured at fair value and other measurements	1,361,768	1,859,456
Derivatives	10,722	13,712
Contingent payment arrangements	251,522	247,309
Total liabilities measured at fair value	262,244	261,021
Recurring | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	7,262	7,750
Recurring | Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	77,433	42,526
Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	143,560	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	9,959	8,175
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	171,192	95,521
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	2,945	1,768
Total investments	123,880	136,344
Total assets measured at fair value	298,017	233,633
Derivatives	10	162
Contingent payment arrangements	0	0
Total liabilities measured at fair value	10	162
Level 1 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	116,893	129,655
Level 1 | Recurring | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	6,987	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)	927,934	1,521,705
Derivatives	21,483	22,944
Total investments	25,077	27,799
Total assets measured at fair value	974,494	1,572,448
Derivatives	10,712	13,550
Contingent payment arrangements	0	0
Total liabilities measured at fair value	10,712	13,550
Level 2 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	25,077	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	122	129
Total assets measured at fair value	122	129
Derivatives	0	0
Contingent payment arrangements	251,522	247,309
Total liabilities measured at fair value	251,522	247,309
Level 3 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	122	129
NAV Expedient | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	1,468	1,484
Total assets measured at fair value	1,468	1,484
NAV Expedient | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 1,468	$ 1,484