Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Consolidated VIEs
Trading
Derivatives	$ 58,999
Securities sold not yet purchased
Derivatives	57,467
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	115,570	$ 116,445
U.S. Treasury Bills	634,440	485,121
Available-for-sale
Equity securities	203	181
Fixed income securities	102	183
Trading
Equity securities	256,668	326,235
Fixed income securities	130,365	182,776
Long exchange-traded options	8,045	5,910
Derivatives	7,007	12,588
Private equity	4,831	30,340
Total assets measured at fair value	1,461,579	1,159,779
Securities sold not yet purchased
Short equities – corporate	12,944	15,254
Short exchange-traded options	772	843
Derivatives	9,548	12,221
Contingent payment arrangements	30,861	31,399
Total liabilities measured at fair value	111,592	59,717
Recurring | Consolidated VIEs
Securities sold not yet purchased
Derivatives	57,467
Recurring | Consolidated VIEs | Investments
Consolidated VIEs
Investments	247,021
Recurring | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	57,327
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	115,570	116,445
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	203	181
Fixed income securities	102	183
Trading
Equity securities	255,744	304,083
Fixed income securities	129,676	180,194
Long exchange-traded options	8,045	5,910
Derivatives	827	1,539
Private equity	0	14,305
Total assets measured at fair value	611,271	622,840
Securities sold not yet purchased
Short equities – corporate	12,944	15,254
Short exchange-traded options	772	843
Derivatives	1,750	2,651
Contingent payment arrangements	0	0
Total liabilities measured at fair value	15,908	18,748
Recurring | Level 1 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	442
Recurring | Level 1 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	100,619
Recurring | Level 1 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	485
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
U.S. Treasury Bills	634,440	485,121
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	817	22,039
Fixed income securities	689	2,582
Long exchange-traded options	0	0
Derivatives	6,180	11,049
Private equity	0	0
Total assets measured at fair value	833,657	520,791
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	7,798	9,570
Contingent payment arrangements	0	0
Total liabilities measured at fair value	64,823	9,570
Recurring | Level 2 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	57,025
Recurring | Level 2 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	134,689
Recurring | Level 2 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	56,842
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	107	113
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	4,831	16,035
Total assets measured at fair value	16,651	16,148
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	30,861	31,399
Total liabilities measured at fair value	30,861	$ 31,399
Recurring | Level 3 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	0
Recurring | Level 3 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	11,713
Recurring | Level 3 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	$ 0