Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Securities sold not yet purchased
Contingent payment arrangements	$ 0	$ 9,365	$ 17,100
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	116,445	89,566
U.S. Treasury Bills	485,121	444,152
Available-for-sale
Equity securities	181	5,951
Fixed income securities	183	221
Trading
Equity securities	326,269	387,502
Fixed income securities	182,788	167,059
Long exchange-traded options	5,910	22,290
Derivatives	12,588	6,787
Private equity	62,407	71,088
Total assets measured at fair value	1,191,892	1,194,616
Securities sold not yet purchased
Short equities – corporate	15,254	81,784
Short exchange-traded options	843	7,118
Derivatives	12,221	9,392
Contingent payment arrangements	31,399	42,436
Total liabilities measured at fair value	59,717	140,730
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	116,445	89,566
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	181	5,951
Fixed income securities	183	221
Trading
Equity securities	304,083	387,495
Fixed income securities	180,194	164,317
Long exchange-traded options	5,910	22,290
Derivatives	1,539	571
Private equity	14,305	12,162
Total assets measured at fair value	622,840	682,573
Securities sold not yet purchased
Short equities – corporate	15,254	81,784
Short exchange-traded options	843	7,118
Derivatives	2,651	2,438
Contingent payment arrangements	0	0
Total liabilities measured at fair value	18,748	91,340
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury Bills	485,121	444,152
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	22,070	7
Fixed income securities	2,594	2,742
Long exchange-traded options	0	0
Derivatives	11,049	6,216
Private equity	0	0
Total assets measured at fair value	520,834	453,117
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	9,570	6,954
Contingent payment arrangements	0	0
Total liabilities measured at fair value	9,570	6,954
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
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	116	0
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	48,102	58,926
Total assets measured at fair value	48,218	58,926
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	31,399	42,436
Total liabilities measured at fair value	$ 31,399	$ 42,436