Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Consolidated VIEs
Trading
Derivatives	$ 66,227
Securities sold not yet purchased
Derivatives	66,812
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	1,646,335	$ 1,159,825
Assets, Fair Value Disclosure [Abstract]
Money markets	249,818	116,445
U.S. Treasury Bills	664,857	485,121
Available-for-sale
Equity securities	186	181
Fixed income securities	107	183
Trading
Equity securities	262,744	326,269
Fixed income securities	138,867	182,788
Long exchange-traded options	4,709	5,910
Derivatives	9,155	12,588
Private equity	6,500	30,340
Securities sold not yet purchased
Short equities – corporate	10,143	15,254
Short exchange-traded options	3,328	843
Derivatives	15,385	12,221
Contingent payment arrangements	31,119	31,399
Total liabilities measured at fair value	126,787	59,717
Recurring | Consolidated VIEs
Securities sold not yet purchased
Derivatives	66,812
Recurring | Consolidated VIEs | Investments
Consolidated VIEs
Investments	243,165
Recurring | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	66,227
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	729,922	622,840
Assets, Fair Value Disclosure [Abstract]
Money markets	249,818	116,445
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	186	181
Fixed income securities	107	183
Trading
Equity securities	259,595	304,083
Fixed income securities	138,279	180,194
Long exchange-traded options	4,709	5,910
Derivatives	373	1,539
Private equity	0	14,305
Securities sold not yet purchased
Short equities – corporate	10,143	15,254
Short exchange-traded options	3,328	843
Derivatives	2,957	2,651
Contingent payment arrangements	0	0
Total liabilities measured at fair value	16,876	18,748
Recurring | Level 1 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	448
Recurring | Level 1 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	76,678
Recurring | Level 1 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	177
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	894,382	520,834
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury Bills	664,857	485,121
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	3,044	22,070
Fixed income securities	588	2,594
Long exchange-traded options	0	0
Derivatives	8,782	11,049
Private equity	0	0
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	12,428	9,570
Contingent payment arrangements	0	0
Total liabilities measured at fair value	78,792	9,570
Recurring | Level 2 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	66,364
Recurring | Level 2 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	151,061
Recurring | Level 2 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	66,050
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	22,031	16,151
Assets, Fair Value Disclosure [Abstract]
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	105	116
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	6,500	16,035
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	31,119	31,399
Total liabilities measured at fair value	31,119	$ 31,399
Recurring | Level 3 | Consolidated VIEs
Securities sold not yet purchased
Derivatives	0
Recurring | Level 3 | Consolidated VIEs | Investments
Consolidated VIEs
Investments	15,426
Recurring | Level 3 | Consolidated VIEs | Derivatives
Consolidated VIEs
Investments	$ 0