Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 66,649	$ 73,649
Securities sold not yet purchased
Contingent payment arrangements		17,400
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	123,767	126,401
Securities segregated (U.S. Treasury Bills)	2,012,198	1,094,866
Derivatives	32,566	13,491
Total investments measured at fair value and other measurements	262,508	266,794
Total assets measured at fair value and other measurements	2,431,039	1,501,552
Securities sold not yet purchased
Short equities – corporate	11,745	17,809
Short exchange-traded options	15,884	12,348
Derivatives	40,056	19,259
Contingent payment arrangements	23,704	22,911
Total liabilities measured at fair value	91,389	72,327
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	123,767	126,401
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,505	939
Total investments	177,993	183,760
Total assets measured at fair value	303,265	311,100
Securities sold not yet purchased
Short equities – corporate	11,745	17,809
Short exchange-traded options	15,884	12,348
Derivatives	4,895	871
Contingent payment arrangements	0	0
Total liabilities measured at fair value	32,524	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)	2,012,198	1,094,866
Derivatives	31,061	12,552
Total investments	17,589	8,952
Total assets measured at fair value	2,060,848	1,116,370
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	35,161	18,388
Contingent payment arrangements	0	0
Total liabilities measured at fair value	35,161	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	118	119
Total assets measured at fair value	118	119
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	23,704	22,911
Total liabilities measured at fair value	23,704	22,911
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	159	314
Equity securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	154,439	180,331
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	136,573	170,946
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	17,589	8,952
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	118	119
Equity securities | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	159	314
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	6,953	8,007
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	11,590
Total investments measured at fair value and other measurements		13,890
Other investments | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	4,637	5,883
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	17,303	18,281
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	17,303	18,281
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	36,783	6,931
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	36,783	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	42,393	47,361
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 42,393	$ 47,361