Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	$ 37,100
Other	56,058	$ 54,229
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	48,942	62,071
Securities segregated (U.S. Treasury Bills)	1,024,529	816,350
Derivatives	13,565	11,619
Private equity	38,011	38,186
Investments, NAV Expedient	37,470	37,326
Total investments measured at fair value and other measurements	405,381	443,589
Total assets measured at fair value and other measurements	1,492,417	1,333,629
Securities sold not yet purchased
Short equities – corporate	13,567	16,376
Short exchange-traded options	13,966	13,585
Derivatives	13,106	14,323
Contingent payment arrangements	10,908	10,855
Total liabilities measured at fair value	51,547	55,139
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	48,942	62,071
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,517	948
Private equity	0	0
Total investments	242,564	281,736
Total assets measured at fair value	293,023	344,755
Securities sold not yet purchased
Short equities – corporate	13,567	16,376
Short exchange-traded options	13,966	13,585
Derivatives	1,240	2,540
Contingent payment arrangements	0	0
Total liabilities measured at fair value	28,773	32,501
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)	1,024,529	816,350
Derivatives	12,048	10,671
Private equity	0	0
Total investments	68,218	69,227
Total assets measured at fair value	1,104,795	896,248
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	11,866	11,783
Contingent payment arrangements	0	0
Total liabilities measured at fair value	11,866	11,783
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
Private equity	954	954
Total investments	1,071	1,071
Total assets measured at fair value	1,071	1,071
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	10,908	10,855
Total liabilities measured at fair value	10,908	10,855
U.S. Treasury Bills | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	52,685	52,609
U.S. Treasury Bills | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
U.S. Treasury Bills | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	52,685	52,609
U.S. Treasury Bills | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Equity securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	253,232	293,584
Investments, NAV Expedient	413	94
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	237,169	276,755
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	15,533	16,618
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	117	117
Private equity | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	37,057	37,232
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	11,786	11,892
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	11,786	11,892
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	5,160	5,138
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	5,160	5,138
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,395	4,981
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,395	4,981
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	39,112	37,199
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 39,112	$ 37,199