Fair Value - Valuation of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 65,296	$ 73,649
Securities sold not yet purchased
Contingent payment arrangements	4,400		$ 17,384
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	126,249	126,401
Securities segregated (U.S. Treasury Bills)	1,882,188	1,094,866
Derivatives	24,224	13,491
Total investments measured at fair value and other measurements	294,666	266,794
Total assets measured at fair value and other measurements	2,327,327	1,501,552
Securities sold not yet purchased
Short equities – corporate	6,541	17,809
Short exchange-traded options	9,379	12,348
Derivatives	26,626	19,259
Contingent payment arrangements	28,910	22,911
Total liabilities measured at fair value	71,456	72,327
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	126,249	126,401
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	113	939
Total investments	209,657	183,760
Total assets measured at fair value	336,019	311,100
Securities sold not yet purchased
Short equities – corporate	6,541	17,809
Short exchange-traded options	9,379	12,348
Derivatives	1,126	871
Contingent payment arrangements	0	0
Total liabilities measured at fair value	17,046	31,028
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)	1,882,188	1,094,866
Derivatives	24,111	12,552
Total investments	19,425	8,952
Total assets measured at fair value	1,925,724	1,116,370
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	25,500	18,388
Contingent payment arrangements	0	0
Total liabilities measured at fair value	25,500	18,388
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	114	119
Total assets measured at fair value	114	119
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	28,910	22,911
Total liabilities measured at fair value	28,910	22,911
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	174	314
Equity securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	211,544	180,331
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	191,831	170,946
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	19,425	8,952
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	114	119
Equity securities | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	174	314
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	7,868	8,007
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	13,543	13,890
Other investments | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	5,675	5,883
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	17,418	18,281
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	17,418	18,281
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	12,151	6,931
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	12,151	6,931
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	40,010	47,361
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 40,010	$ 47,361