Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Securities sold not yet purchased
Contingent payment arrangements	$ 0		$ 9,365
Recurring
Assets, Fair Value Disclosure [Abstract]
Money markets	261,553	$ 89,566
U.S. Treasury Bills	374,998	444,152
Available-for-sale
Equity securities	135	5,951
Fixed income securities	190	221
Trading
Equity securities	341,132	387,502
Fixed income securities	172,605	167,059
Long exchange-traded options	11,178	22,290
Derivatives	18,739	6,787
Private equity	63,855	71,088
Total assets measured at fair value	1,244,385	1,194,616
Securities sold not yet purchased
Short equities – corporate	26,606	81,784
Short exchange-traded options	4,103	7,118
Derivatives	11,354	9,392
Contingent payment arrangements	38,817	42,436
Total liabilities measured at fair value	80,880	140,730
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Money markets	261,553	89,566
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	135	5,951
Fixed income securities	190	221
Trading
Equity securities	320,051	387,495
Fixed income securities	170,572	164,317
Long exchange-traded options	11,178	22,290
Derivatives	2,740	571
Private equity	14,388	12,162
Total assets measured at fair value	780,807	682,573
Securities sold not yet purchased
Short equities – corporate	26,606	81,784
Short exchange-traded options	4,103	7,118
Derivatives	264	2,438
Contingent payment arrangements	0	0
Total liabilities measured at fair value	30,973	91,340
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury Bills	374,998	444,152
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	20,962	7
Fixed income securities	2,033	2,742
Long exchange-traded options	0	0
Derivatives	15,999	6,216
Private equity	0	0
Total assets measured at fair value	413,992	453,117
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	11,090	6,954
Contingent payment arrangements	0	0
Total liabilities measured at fair value	11,090	6,954
Recurring | Level 3
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	119	0
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	49,467	58,926
Total assets measured at fair value	49,586	58,926
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	38,817	42,436
Total liabilities measured at fair value	$ 38,817	$ 42,436