Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 81,434	$ 77,959
Securities sold not yet purchased:
Contingent payment arrangements	0		$ 7,525
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	122,582	151,156
Securities segregated (U.S. Treasury Bills)	1,692,536	1,503,828
Derivatives	22,082	15,422
Total investments measured at fair value and other measurements	243,483	273,418
Total assets measured at fair value and other measurements	2,080,683	1,943,824
Securities sold not yet purchased:
Short equities – corporate	3,427	1,054
Short exchange-traded options	221	2,774
Derivatives	16,147	16,923
Contingent payment arrangements	39,098	38,260
Total liabilities measured at fair value	58,893	59,011
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	122,582	151,156
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,564	392
Total investments	133,734	155,904
Total assets measured at fair value	257,880	307,452
Securities sold not yet purchased:
Short equities – corporate	3,427	1,054
Short exchange-traded options	221	2,774
Derivatives	1,895	1,186
Contingent payment arrangements	0	0
Total liabilities measured at fair value	5,543	5,014
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	1,692,536	1,503,828
Derivatives	20,518	15,030
Total investments	28,052	39,284
Total assets measured at fair value	1,741,106	1,558,142
Securities sold not yet purchased:
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	14,252	15,737
Contingent payment arrangements	0	0
Total liabilities measured at fair value	14,252	15,737
Recurring | Level 3
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	39,098	38,260
Total liabilities measured at fair value	39,098	38,260
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	144	145
Total assets measured at fair value	144	145
Equity securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	153,743	184,472
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	125,428	144,917
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	28,052	39,284
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	119	126
Equity securities | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	144	145
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	4,767	6,015
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments measured at fair value and other measurements	12,628	15,109
Other investments | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	7,861	9,094
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	21,942	21,024
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	21,942	21,024
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	445	1,893
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	445	1,893
Long | Exchange-traded options | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Long | Exchange-traded options | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	54,725	50,920
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 54,725	$ 50,920