Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 68,509	$ 77,959
Securities sold not yet purchased:
Contingent payment arrangements	2,500		$ 7,800
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	20,210	21,024
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	43,797	50,920
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	4,502	6,015
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	125,126	151,156
Securities segregated (U.S. Treasury Bills)	1,746,873	1,503,828
Derivatives	27,347	15,422
Total investments measured at fair value and other measurements	216,463	273,418
Total assets measured at fair value and other measurements	2,115,809	1,943,824
Securities sold not yet purchased:
Short equities – corporate	0	1,054
Short exchange-traded options	0	2,774
Derivatives	12,524	16,923
Contingent payment arrangements	42,436	38,260
Total liabilities measured at fair value	54,960	59,011
Recurring | Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	20,210	21,024
Recurring | Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	43,797	50,920
Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	141,381	184,472
Recurring | Exchange-traded options | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	59	1,893
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	11,016	15,109
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	125,126	151,156
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,724	392
Total investments	116,921	155,904
Total assets measured at fair value	243,771	307,452
Securities sold not yet purchased:
Short equities – corporate	0	1,054
Short exchange-traded options	0	2,774
Derivatives	348	1,186
Contingent payment arrangements	0	0
Total liabilities measured at fair value	348	5,014
Level 1 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	110,348	144,917
Level 1 | Recurring | Exchange-traded options | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	59	1,893
Level 1 | Recurring | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	6,514	9,094
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)	1,746,873	1,503,828
Derivatives	25,623	15,030
Total investments	30,783	39,284
Total assets measured at fair value	1,803,279	1,558,142
Securities sold not yet purchased:
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	12,176	15,737
Contingent payment arrangements	0	0
Total liabilities measured at fair value	12,176	15,737
Level 2 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	30,783	39,284
Level 2 | Recurring | Exchange-traded options | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
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	119	126
Total assets measured at fair value	119	126
Securities sold not yet purchased:
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	42,436	38,260
Total liabilities measured at fair value	42,436	38,260
Level 3 | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	119	126
Level 3 | Recurring | Exchange-traded options | Long
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
NAV Expedient | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	131	145
Total assets measured at fair value	131	145
NAV Expedient | Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 131	$ 145