Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Consolidated VIEs
Trading
Derivatives	$ 3,968
Securities sold not yet purchased
Derivatives	4,238
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	240,634	$ 116,445
U.S. Treasury Bills	474,401	485,121
Available-for-sale
Equity securities	114	181
Fixed income securities	114	183
Trading
Equity securities	203,072	326,235
Fixed income securities	132,846	182,776
Long exchange-traded options	3,549	5,910
Derivatives	6,944	12,588
Private equity	4,831	30,340
Consolidated VIEs
Total assets measured at fair value	1,275,405	1,159,779
Securities sold not yet purchased
Short equities – corporate	24,895	15,254
Short exchange-traded options	4,321	843
Derivatives	7,050	12,221
Contingent payment arrangements	18,017	31,399
Total liabilities measured at fair value	58,521	59,717
Recurring | Consolidated VIEs
Securities sold not yet purchased
Derivatives	4,238
Recurring | Consolidated VIEs | Investments
Consolidated VIEs
Investments	205,022
Recurring | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	3,878
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	240,634	116,445
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	114	181
Fixed income securities	114	183
Trading
Equity securities	202,063	325,248
Fixed income securities	132,162	180,194
Long exchange-traded options	3,549	5,910
Derivatives	1,197	1,539
Private equity	0	14,305
Consolidated VIEs
Total assets measured at fair value	608,671	644,005
Securities sold not yet purchased
Short equities – corporate	24,895	15,254
Short exchange-traded options	4,321	843
Derivatives	600	2,651
Contingent payment arrangements	0	0
Total liabilities measured at fair value	29,862	18,748
Recurring | Level 1 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	46
Recurring | Level 1 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	28,751
Recurring | Level 1 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	87
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
U.S. Treasury Bills	474,401	485,121
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	900	874
Fixed income securities	684	2,582
Long exchange-traded options	0	0
Derivatives	5,747	11,049
Private equity	0	0
Consolidated VIEs
Total assets measured at fair value	659,035	499,626
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	6,450	9,570
Contingent payment arrangements	0	0
Total liabilities measured at fair value	10,642	9,570
Recurring | Level 2 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	4,192
Recurring | Level 2 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	173,512
Recurring | Level 2 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	3,791
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	109	113
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	4,831	16,035
Consolidated VIEs
Total assets measured at fair value	7,699	16,148
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	18,017	31,399
Total liabilities measured at fair value	18,017	$ 31,399
Recurring | Level 3 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	0
Recurring | Level 3 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	2,759
Recurring | Level 3 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	$ 0