Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 53,124	$ 51,762
Contingent payment arrangements	$ 248,185	$ 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	$ 7,528	$ 7,750
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	42,570	42,526
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	3,026	1,486
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	95,690	95,521
Securities segregated (U.S. Treasury Bills)	1,054,638	1,521,705
Derivatives	16,687	24,712
Total investments measured at fair value and other measurements	211,938	217,518
Total assets measured at fair value and other measurements	1,378,953	1,859,456
Derivatives	16,411	13,712
Contingent payment arrangements	248,185	247,309
Total liabilities measured at fair value	264,596	261,021
Recurring | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	7,528	7,750
Recurring | Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	42,570	42,526
Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	151,920	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	9,920	8,175
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	95,690	95,521
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	4	1,768
Total investments	128,865	136,344
Total assets measured at fair value	224,559	233,633
Derivatives	4,878	162
Contingent payment arrangements	0	0
Total liabilities measured at fair value	4,878	162
Level 1 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	121,971	129,655
Level 1 | Recurring | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	6,894	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)	1,054,638	1,521,705
Derivatives	16,683	22,944
Total investments	28,255	27,799
Total assets measured at fair value	1,099,576	1,572,448
Derivatives	11,533	13,550
Contingent payment arrangements	0	0
Total liabilities measured at fair value	11,533	13,550
Level 2 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	28,255	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	170	129
Total assets measured at fair value	170	129
Derivatives	0	0
Contingent payment arrangements	248,185	247,309
Total liabilities measured at fair value	248,185	247,309
Level 3 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	170	129
NAV Expedient | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	1,524	1,484
Total assets measured at fair value	1,524	1,484
NAV Expedient | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 1,524	$ 1,484